SCPM: Rise of the Triad has hats for each of the five characters on different holidays, including Halloween:
Easter <varies> Lorelei Ni wears Easter Bunny ears.
Cinco de Mayo 05/05 Ian wears a sombrero.
Independence Day 07/04 Doug holds an American flag.
Halloween 10/31 Thi wears a witch's hat.
Christmas 12/24-25 Taradino wears a Santa Claus hat; new music for the first level.
Raptor: Call of the Shadows has changes to the game on certain developers' birthdays, I don't know if it still worked on the version that was available on GOG:
http://www.rinkworks.com/apogee/s/6.3.2.shtml Thank you, I finally had time to test
Raptor: Call of the Shadows, and from what I saw they cut out the Apogee logo, so that part of the Easter Egg is missing, but the extra enemies do appear (I found the monkeys, so I think there are the others in the other levels as well), and in 2 out of 4 cases you hear monkey noises when you exit the game.
I have also updated
Rise of the Triad!
dnovraD: Cave Story has a holiday mode, but you should not engage with the Nicalis version. They are thieves who paid Pixel none.
Thank you added, I know the story unfortunately.