the problem isn't turning on the sprinklers. The problem is that the backflow freezes. Average last date of frost around here is May 15th.

BUT

take some insulation, put it over the backflow (the big ugly copper thing that comes out of your house), put a garbage bag over it (to keep it from getting wet)

good down into the single digits.

Really, I swear to you, used to do sprinklers for years and years.

Just because it's on, doesn't mean you have to turn the clock on yet.

But the longer you wait, 1 the lower your water bill and 2. the better your grass will do in the heat of the summer. Let it stress a little and build some roots.