OK, it is a factor of where you are, climate, elevation, lattitude, etc. Still, the old advice is not too far off.
When the dogwoods start to bloom, the crappie are soon to spawn (see note below).
When the night time temperatures are above 45 degrees for three days and the day time temperatures are above 65 degrees for two days, the crappie should be spawning (see note below).
Note: Deep cold lakes will have spawning crappie much later then shallow warm lakes.