The harvesting time seems to vary from year to year these days!
but there are also north/south variations as well, with the south being earlier.
Speaking from the Newcastle/Gateshead latitude:
last year it was well into April, where as 2006, it was the back end of March.
Following the tip with buds will get you onto the flow when it starts.
A couple of other tips are:
You need to select tree's of at least 30cm's diameter.
Put your tap into the south facing aspect of the tree.
Obviously, make sure you plug the tap hole tightly when your done, keeping the plug as clean as possible.
There are a few references to birch sap knocking around on here including one with my "patent"
sap collection bottle-
http://i181.photobucket.com/albums/x125/stalch/bottleresized.jpg
it's 2 tesco's mountain tap--no Spring-water bottles, with cordage made from pallet wrap and toilet roll wrap, handle plastic!-(buy NOWT!-recycle)
Good luck
R.B.