In which case I will inform them at the beginning of January
I'd let them know a bit earlier (unless you've a reason to think that the tenant would do a runner). If the tenancy agreement has an end date however, this is just a matter of courtesy.
provide alternative accommodation, would that apply in this case?
No. You don't need to provide alternative accommodation.
It the tenants have a meldunek, you'll need to cancel that (though it isn't an urgency and can wait). If the tenants have a meldunek and there's a child under 18 at the address, there can be other issues however.