According to 1-Up, it is tucked away in the "My Dreams" section where you raise A-Life (artificial life) creatures.

January saw Nightopians bring in the new year with a fireworks display every hour after midnight throughout January 1st.

According to here is what is set for the rest of the year:

  • February: Nightopians revisit the New Year theme to celebrate the Chinese New Year. This time you can catch them holding a giant dragon and throwing firecrackers.
  • March: March is the month of carnivals, and as such many Nightopians will don a mask for the occasion.
  • April: For this month the Nightopians show off their passion by wearing a Flamenco-inspired rose.
  • May: The Japanese celebrate the Boys' Festival on May 5, and so do your Nightopians. This month they can be seen flying a carp streamer.
  • June: Acknowledging the tradition of June weddings, Nightopians will appear with a boquet or flower basket.
  • July: Nightopians equip themselves with bull armor and a red cape to celebrate the Bull-Running Festival.
  • August: Nightopians turn to music, performing with various instruments during this month.
  • September: The Nightopians transition into fall with full moon paper fans.
  • October: Nightopians will dress up as witches for Halloween and will also carry pumpkin lanterns throughout this month.
  • November: Autumn leaves and fruit make for a perfectly nationality-free Thanksgiving for the Nightopians.
  • December: Here's where the Nightopians really go nuts. Throughout December, the Nightopians will dress as Santa and join together under a Christmas tree. They can also be seen carrying around cake, wearing a tricorn, and letting off fireworks. On December 24 and 25 specifically the Nightopians will sing a special Christmas song.

If you were wondering how all this excellently fun stuff works - the game uses the Wii Calendar to determine the date. Theoretically this means if you're willing to mess around with the calendar you can watch all these events without having to wait for the new month. But that kind of takes way the fun doesn't it?