You can get pre-approved any time. Depending on the terms, they hold the rate for you for 30-120 days (or more). Usually people get pre-approval when they are thinking about buying a house (so before purchasing), so they know roughly what price range is feasible. Some people might get pre-approved now with the intention of buying this summer since interest rates are reportedly set to rise soon.
Technically you can get pre-approved even if you aren't even thinking of buying a place.
EDIT: after you buy a (resale) place, you get approved/finalize the mortgage - you have to submit info about the property to your lender and they have to okay everything. Pre-approval is for when you are shopping around for a place. The rules are different though for new condos that are not yet registered.