Moderator, Marriott Bonvoy & FlyerTalk Evangelist
Join Date: Oct 2002
Location: McKinney, TX, USA
Programs: United Silver; AA Plat/2MM; Marriott LT Titanium; Hilton Gold
Posts: 11,774
The stay must occur before the cert expires. Once the Cert expires, it essentially disappears from the Marriott system and any reservation you had with it attached will no longer have a cert attached. So if the stay hasn't occurred yet, then you might find Marriott's system suddenly pulling points from your account a week before your stay.