Cave Story+ has a number of extras that only trigger on certain dates. Keep in mind that this only happens in Cave Story+ and not the original.
Triggers on: The 26th of October to the 2nd of November.
All the in-game graphics change to a Halloween theme.
Triggers on: The 24th of December to the 6th of January.
All the in-game graphics change to a Christmas theme.
Triggers on: The 29th of April.
If the soundtrack is set to Cave Story+ or New then the Ikachan theme song will start playing on the title screen.