The only time the pools close outside of their "regular hours" which are at least until around 5:00pm every day for the main pool and generally 24 hours but no lifeguard for the quiet pool(s) is when there's lightning.
Decembre can go in either direction. The earlier in the month you are going the warmer the weather will likely be.
During the day it will ikely be pleasant, but especially later in the omnth expect it to be COLD at night. (Even frost!)
Anne