a few thoughts. first, be cautious of what you find on the internet - as with anything, there's a lot of unreliable info out there. that said, it really depends on what you're trying to accomplish with your pruning. if you prune just before budbreak as chef robin suggested, the overall effect will be to encourage the remaining limbs by giving them more access to nutrients and water. you can do that to invigorate existing limbs, and you will influence the future growth pattern of the tree in the process. on the other hand, if you want to inhibit growth and keep the tree at its present size, you'd want to do that just after the plant completes its major yearly growth (ie. probably around september).

pruning before budbreak will redirect the growth of the limbs that you prune. if you cut off the end of a branch, it will grow more lateral stems and become more bushy. to do that, you should make your pruning cuts just above budding points or just above the emergence points of lateral stems. if you cut off the lateral stems, the branch will become more long and spindly. generally, don't remove more than about 25-30% max of the canopy in one go or you'll end up stressing the tree by creating an imbalance between shoot and root biomass.

i hope that helps. if you want to get heavy into this, i'd highly recommend the book Arboriculture by harris, clark, and matheny. it's a textbook quality resource, but written in a very accessible manner. good luck.