Hi Mr. Cohen

You don't say what type of preemergent herbicide you are looking to use or what you are using it for- but I'm going to guess you are looking to control crabgrass in your turf?  If this is the case, in Colorado that time is mid to end of March.  Crabgrass germinates from mid April to mid May, so the preemergent would be applied 2-4 weeks prior.  Here is Fact Sheet 3.101 which is about controlling grassy weeds in your lawn.

If this was not the information you were seeking, please reach out again.

Master Gardener Volunteer