In the iOS App (same on PC/Chrome) you can get the info regarding the closing date by touching the info icon (circled i) in each race tile on the home screen of the game.But... you need to finish before they close the race. If they close it while you're still at sea, you'll get nada. And they haven't been announcing the closure date. So don't kill yourself farming cards and then lose your crop by failing to finish at all.
Closing dates:
Mini Transat: 11/22
Transat Jacques Vabre: 11/23
VOR leg 2: 12/05
Clipper leg 3: 11/27