I open my pool in the spring before algae starts growing. Water temperature is also a factor. This year I opened early in May even though we didn't swim until the end of the month. Last year we had a heat wave early and I actually opened in April. It's much easier to maintain a clean pool than it is to fix a green pool. Plus maintainance is pretty easy early in the year when the sun is low in the sky and the pool doesn't use up FC quickly. My pool gets a lot of shade as well, especially early in the season.
It surprises me how many posts I see here in mid-June from people saying they just opened, their pool is a green swamp, and they have a pool party scheduled in two days. Proper maintenance is what keeps a pool from turning green, and you have to do that maintenance when the pool demands it. Ignore the pool and you get a swamp.