Which time to use to mark ""when" it happened: for school murders, you have a limited amount of hours in the day in which to do it, and only 5 days of the week.
Wouldn't a more crucial time be when the decision was firmly made that that day and that hour would be the time? If a periodic outside influence was going over the school site at the time of the shootings, it still might not be as crucial as he had to spend time the day beforehand deciding and preparing for it.
Mar/April: Check Easter dates for correlation, and Spring or Easter break dates for each particular school that year. For years, our school district usually started spring break on Good Friday, and had it go for the week after Easter. But recently they've been having it be the week before Easter, possibly because they don't have to take Good Friday off as a separate holiday that way. If there were multiple perpetrators, being able to meet for a few days during break might be a consideration.
There's been some correlation with sidereal time being correlated with with a rise in psi ability. If you had the time of the decisions, that might be possibly relevant.