This is a hard question because the answer will vary year to year. 

April 15th is the average "safe date" for sprinkler start-ups and is the guide point we use for planning our start-up season.

We want about two weeks of nighttime temperatures above 32 degrees to allow things below ground to thaw.

Often "false spring" will give us a few glorious, sunny days and you will be tempted to fire up your system. If the ground temperature is still cold (extension.agron.iastate.edu/npknowledge/soiltemphistory.html) and your valve box(es) are in the shade, your sprinkler likely won't run because there is still ice in the lines/valves.

Keep in mind, if you do get you sprinkler live before winter has truly loosened its grip you will need to keep an eye on the temperatures because any dip below 32 degrees can cause damage to your sprinkler.