As others have said, late December tends to be the main sale (around the 22nd), see
this thread for historical details
Probably unlikely to save much vs a sale fare from AMS or others. Plus if you get to use the lounge at T5, then that tends to be worth a bit extra IMO. Plus there's the potential opportunity to purchase a cheap upgrade to Club (often offered online in 'manage my booking' shortly before the flight if there's availability).