There's a good article on trifuel on peaking for a race. I disagree there is "much confusion" (in the first paragraph) but it is a good article on tapering and peaking for your race which you DO need to plan in advance.

The competitive season for triathlons is typically May through September. Within that period you might have a race you really want to do well in, often called your "A" race, and those used more for tracking progress.