Water temperatures approaching the low 60's and the lengthening daylight hours are usually the main triggers.
IMHO ... the Moon is always "full" (the same size) ... it's just shadowed (partially or fully) to some degree, except during a "Full Moon". That being said, the Full Moon extends the time length of light shining on the water, and may provide a triggering effect. But, a full moon and longer periods of light do not trump water temperature. And in most cases, the spawn does not take place over the period of a couple of days ... it can & usually does take a few weeks, from start to finish.
Even water temps have a "range" that's suitable for the Crappie to spawn in. They don't just wait for a magical degree to arrive. Some will be trying to spawn in the mid to upper 50deg water, and others will wait on low to mid 60deg water.
If there's one thing I've learned in my 45yrs of fishing, it's that there are no "absolutes" ... there are only "ranges" & "trends", and even they can change, sometimes slightly & sometimes drastically.
... cp