3:15 for lcalcron.php

Ha ha, yes great question, why exactly at 3:15am? Actually it does not matter so much when lcalcron.php is started.
Since this file triggers the sending of email reminders for the day, it should start before the early rising calendar users open the calendar.
So why not start lcalcron.php just after midnight, or at 1:00am? Because various countries have Summer and Winter times (DST) and the DST change is not always at the same hour in each country. If you would start the lcalcron.php file for instance at 2:00am and at the end of the Summer at 2:00am  the clock is set 1 hour back, during that night it will be 2:00am twice! This would then start the lccron.php script twice resulting in duplicate reminder messages.
So, to make a long story short, we said let's suggest 3:15am; this will not interfere with DST changes in the various countries and it's still early enough to send reminders for the day to come.
I hope this sufficiently explains the 3:15am mystery
Roel