"Wæs þu hæl" is an Anglo Saxon toast meaning "be thou hale" ("be in good health"). The toast, if not the customs which the term has come to be associated with, is thought to date from the early eleventh or late tenth century, at least.
There are two kinds of Wassailing - the first of which has come to be closely associated with Christmas and carolling. Wassailers call at people's homes then offer a song and a drink of warmed, spiced ale or cider from a Wassailing bowl (or cup) to the answerer in exchange for money or gifts.
The second originates in the South West of England ("the West Country"), where apple orchards were already providing cider for the thirsty population by the time our Roman invaders arrived.  Today the UK drinks more cider (by that I mean what North Americans refer to as Hard Cider - not mere apple-juice) than anywhere else in the world with the beverage being produced not just in the West Country but also in places like Herefordshire, Worcestershire, Kent, Suffolk, Norfolk, Buckinghamshire, and Cheshire, as well as in Wales and Northern Ireland.
Apple Howling and Apple Wassailing may once have been two distinct practices that have become entwined and conflated over the years. The former was once most commonly performed at New Year and was described by the American naturalist Henry David Thoreau in his 1862 essay "Wild Apples" as follows:
A troop of boys visited the different orchards and, encircling the apple tress, repeated the following words: -
"Stand fast, root! bear well, top!
Pray God send us a good howling crop:
Every twig, apples big;
Every bough, apples enow!"
"They then shout in chorus, one of the boys accompanying them on a cow's horn. During this ceremony they rap the trees with their sticks". 
Apple Wassailing on the other hand most commonly takes place on Twelfth Night (which on Today's calendar is January the 17th). Though the "Stand fast, root" incantation and the rest of the Apple Howling custom often forms a part of the Wassailing, there is a lot more to it.
Though it varies not just from county to county, but from village to village, the basics of Apple Wassailing are as follows: The Wassailing party gather in an orchard bringing plenty of cider with them. Toasted bread is soaked in the cider and pieces of it hung from the branches of the trees as an offering to those beneficial spirits which reside within the orchard, and also the birds which might peck at the new buds in the spring. The group gather around the largest, oldest apple tree and a Libation of cider is poured upon its roots. The leader of the party then fires a shotgun into the branches of the tree to scare away any malevolent spirits (and presumably birds, once again). Toasts of cider are then drunk to the tree and a Wassailing Song is sung.
Norton Priory in Runcorn, Cheshire (about ten miles East of my home in Liverpool) was founded in 1134 AD. It was home to an Order of Augustinian Canons and saw several phases of building and rebuilding, culminating in 1391 with its elevation to Abbey status. The Abbey met its end in April 1536 during the first phase of Henry VIII's dissolution of the monasteries. The buildings and estate were bought by the Brooke family who lived there up until the early twentieth century.
Today, much of the original layout of the buildings - the cloisters, church, refectory and dormitory can still be seen. There is also an intact 12th century undercroft. A two and a half acre Walled Garden has been recreated as it would have been in Georgian times. There is an orchard, a trained fruit garden, a vegetable garden, various ornamental borders and the beautiful Rose Walk. 
This year Norton Priory held its first recorded Apple Wassail on the 17th of January. Archaeologist, folklorist, historian, storyteller, and bagpiper Tom Hughes is the man responsible for bringing the custom to the priory and he was good enough to answer a few questions about it for me.
First, I asked how the whole thing came about.
I'd personally been involved in wassailing at Stretton Watermill in Cheshire for the past four years when I was working there. Some of the Norton rangers and gardeners had come along to that and when I started work at Norton Priory on a project to develop a new museum they asked me if I would help get a wassail going there. I was only too pleased to do so! There is a nice association as well as the mistletoe we managed to get established on the apple trees at Stretton Mill came from berries in the Norton Priory walled garden.
Next I asked Tom to take us through the Appple Wassail as it occurred, step by step.
We decided to gather at 4pm so that by the time we got to the trees it would be getting dark and we'd enjoy the atmosphere of the lantern lit orchard. We'd arranged a workshop for families just before, so they could make their own willow and paper lanterns. The wassail was arranged as a thank you to our supporters and volunteers, around eighty people in total, most of whom had never encountered a wassail before, though several had heard one on The Archers omnibus that morning! We began with getting together in a straw bale building beside the Georgian walled garden and enjoyed mulled cider and spiced apple juice. We also passed around the wassail cup. With it being Old Twelfth Night we enjoyed a mummers play*, then I told the story of the Apple Tree Man**. The group then picked up drums, rattles and tambourines and set off on procession through the gardens to the orchard. I led the way, playing the tune of our wassail song as we went.
When we arrived in the orchard inside the garden we had baskets of toast for people to hang in the trees to encourage the good spirits, we have twelve or so apple trees there, mostly old Cheshire varieties, but people got quite carried away and so plums, quince and medlar trees also benefitted! We then sang the Apple Tree Wassail to the oldest tree. The version we used has a tune very similar to the old Cheshire song "Miller of Dee", the words were the version
"Oh apple tree,
We wassail thee,
And hope that thou shall bear.
For the Lord doth know where we shall be,
to be merry another new year.
For to bear well, and to share well,
So merry may we be.
Let every man raise up his cup and shout health to the old apple tree."
We then poured cider on the roots and then made lots of noise with the rattles and drums and several blasts of a shotgun through the branches.
Then we processed back to the straw bale building, this time I played the tune of the Gloucestershire Wassail. We finished with more cider, potatoes baked in the cob oven, and some more storytelling. Everyone was demanding we do it again next year.
Finally, I asked Tom what inspired him to bring what is usually considered a West Country tradition to Cheshire, and what it means to him personally.
Although apple tree wassailing is best known from the West Country, it did take place all along the Welsh borders, up through Shropshire and into Cheshire. Heading along to Norton is perhaps straying a bit east, but we felt the wassailing of the Cheshire variety trees and the use of the local tune made it as much part of our heritage.
I don't know that apple tree wassailing is a very ancient tradition, perhaps 19th or maybe 18th century. Maybe it's older, but I haven't seen proof. I feel it's one of those traditions that folklorists used to want to push back to "pre-Christian" times, rebirth of the seasons and all that, like the old books of the 1970s used to say of morris dancing and mummers. We'd tried to be true to images of older wassails along with taking the best bits of revivals. In the past decade I've enjoyed wassails at a community orchard over in Cambridge and the huge wassail festival at Chepstow. What works best is its way that it brings a community together in the depths of winter and I can imagine that was always the point of it.
I'll raise a cider to that!
* Mummers Plays are seasonal British folk plays, performed by troupes of amateur actors known as mummers or guisers (or by local names such as rhymers, pace-eggers, soulers, tipteerers, wrenboys, galoshins, guysers, and so on). Tom is himself a member of the Jones' Ale Soul Cakers - a group which started out in 1970, taking their name from the Jones' Ale folk club in Chester where they were regulars. The play they perform is taken from the Alderley Mummers script which supposedly dates from 1788.
** The Apple Tree Man is a folktale from Somerset. The Apple Tree Man is the spirit of the oldest apple tree in an orchard, and in whom the fertility of the orchard is said to reside. In the tale a man offers his last mug of mulled cider to the trees in his orchard and is rewarded by the Apple Tree Man who reveals to him the location of buried treasure.
 Marc Alexander, A Compendium of Folklore, Myths & Customs of Britain, Sutton Publishing 2002