I played the game this morning (December 22nd) and was greeted with this message:

CHRISTMAS DETECTED! [y/n]

Pressing Y enabled a chirstmas mode with candy cane bullets and replacement sound and other graphics. Twas a lot of fun.

I've seen this message when I was using wine, but I didn't even notice the christmas theme, 'cause I was worried about the buggy screen format.

Now I'm playing the game through the native DOSBox version for Linux and I'm not given access to this easter egg e.e

Yuup! It's possible right now :)

Another great game, Raptor, has date specific easter eggs too! I think Xwing CD does something on Christmas as well.

Fun!

Wait, THAT Raptor? Call of the Shadows? I should play it on "special" dates to check that.... Do you know what are the specific dates?

By the way, I remember finding out about Christmas mode as a kid on the demo... fun times!

https://www.vogons.org/viewtopic.php?f=5&t=43139

According to posts on that forum the dates are:

May 16, 1994
Feb 27, 1996
Oct 02, 1995
Mar 12, 1995
Aug 28, 1995
Apr 24, 1996