Hello. I'll stay in Issy les Molineaux and I want to go to Disneyland by RER on 25th of December. Which is the nearest station to go? Does RER on function on Christmas day? Thanks in advance.

This trip will require two transfers, but can be done by RER and Metro, both of which run normally on Christmas day.
 
The first step is taking the RER C train at Issy station in Issy-les-Moulineaux towards Paris which means either Massy-Palaiseau or Versailles-Chantiers terminus stations will be listed on the overhead direction signs (either will work).  Get off the RER C at Invalides station and transfer to Metro line 8.
 
Take Metro line 8 in direction of Créteil until Opéra station.
 
At Opéra you'll transfer to RER A in direction Marne-la-Vallée and ride it until the terminus which is at Disneyland Paris.
 
 
This is a rather lengthy trip taking about 1h30m to complete, but a single RER ticket purchased at the beginning at Issy station will cover you the complete journey costing 5€.  Keep using this ticket when making your transfers, putting it into gates/turnstiles and collecting it from the top of these machines.