My Calendar icon does this too. Restarting the phone fixes it temporarily. I restarted on Saturday, and it's Monday the 18th now, but the icon still says Saturday 16. This is annoying. I don't need a reminder of the day I last restarted my phone, I don't think... lol. It would be somewhat less annoying if it didn't even seem to make an attempt at showing a real date. But since it does seem to be trying, I think it should work consistently, not just for the rest of the day I last restarted my device.
Hoping someone finds a more permanent solution than restarting a device every morning or midnight just to get the date right.
Edit... I just tried changing the Theme I was using (MIUI Dark) and the new theme just had a '31' icon. When I switched back to MIUI, the calendar icon updated. So a complete restart is not needed, just a complete resetting of the theme settings apparently (it didn't remember my selected backgrounds, reverted to default)