Crab grass preventer

Usually at the end of April, but this year may be earlier. There are two good indicators of when to put down pre-emergent. One is soil temperature. Use an inexpensive soil thermometer and put down the preventer when the soil temperature one to two inches down is 55 degrees over several days:
http://blog-yard-garden-news.extension.umn.edu/2015/04/spring-preemergent-applications-for.html

Second is to use what's in bloom around you. Put down the pre-emergent when the forsythia is in full bloom:
http://counties.uwex.edu/outagamie/files/2010/05/Crabgrass.pdf