There's christmas, at 10.30pm on christmas eve santa comes and gives you a present
On easter, there's various bunny/egg animations (Mid-march to start of april IIRC)
On valentines you can get a choco heart in the shop
Unconfirmed dates:
Haloween: you can get a pumpkin in the shop on that day
Thanksgiving: (November 25) you can get a turkey running around on your screen
Not sure if there are any more. Also if I didn't answer your question right please say so and I'll try to answer again wasn't quite sure what you meant about 'dates'
Yes i know 1 more! I'm pretty sure that on your birthday your tamagotchi does a little celebration thing for you, i think my V4 did it for me once but that was a while ago....