In XY yes this is true. Whatever season you transfer it that is the form it will stay even the offspring.
I got all four so I can breed you one real quick (if you don't care about IVs).
Also, if anyone happens to have an extra, I'm looking to get a Celebrate event Eevee[Birthday Eevee from X/Y event over in Japan].
edit: Also I do want to give Yogurt a chance to accept my offer before I trade the darkrai to somebody else.
Oh, and here's a list of my other extra event pokemon:
Plasma Deoxys[Unraised] x 1
GAMESTP Jirachi[Came at Lvl 100] x 1
FEB2012 Mewtwo[Unraised] x 1
FAL2010 Mew[Unrasied] x 1
Looking to trade for a chespin or a froakie!
Which one? I can get you both
I have all pokes in my PC so I can breed anyone stuff if anyone wants me too.