A guy from Florida suggested spraying in May, but what if someone is from Minnesota? May won't work--the Minnesota guy could still have snow. And time of spraying also depends on the chemical.
Suggestions work best when they refer to the stage of plant growth. For example, the PNW weed handbook says to spray 2,4-D at the early bud stage. But for glyphosate, it says to spray when plants are actively growing after the bud stage.