No clue, but perhaps we can reason some things out.

1 - Her father left her to die of exposure. Now, a vulnerable infant could be in danger almost any time of the year, but on nights where it could get cold it would be an even greater danger.

2 - She, Kouta and Yuka first meet again (first for Yuka) sometime in late March early April (start of Japan's school year, so I'm told) and she is killed roughly a year after this, since the story concludes soon after the assault on Maple House, which came soon after a snowfall depicted in C72.

3 - It's pretty much agreed that the grand-ancestor of all Magical Girlfriends, with a temper almost as ferocious as Lucy's when provoked, is Mrs. Anthony Nelson, aka Jeannie from I Dream Of Jeannie. Using a super-computer they had limited access to, Major Nelson and Major Healey (goofball friend Roger, later on Bob Newhart) calculated Jeannie's date of birth based on an astronomical event and her probable age range.

That birthday was April 1st, and I hereby suggest it for Lucy. BLINK!