In our town there are two sessions. Last summer it was so hot, that it really did not matter, and there was only one day that was rained out. One 4 week 2 day a week sesson in June and one in July. I put my son in both.
You have to decide your goal. Are the swim lessons at the beginnng going to be a good refresher so your kids can swim the summer? Are they just start lessons, so this is just getting them used to swimming? How much swimming do you do?
I was going to wait until the July session for my daughter (will be two in June) because she acted like you were putting a cat in acid when we were in Fl in March, but we decided to try it. So both my kids start June 4.
So, I guess the bottom line is the goal. For my son, I want him to remember how to swim, so I can supervise him in the pool, not always need to be in the pool right next to him. For my daugther, I just want her to start learing some of the basics and not be afraid of the water.
Good Luck.