Gifts of money that arrive before wedding

BinxRose

My FI's uncle and his family will not be able to attend our wedding, and with the RSVP, wrote a really heartfelt note appologizing. Along with that, they sent a very generous check for a wedding gift.

My question is, what is the protocol on cashing it and writing the thank you note? Should I cash it now so their checkbook is correct, and write a note right now? Should I cash it now, and write a note after the wedding? Should I wait until the wedding to cash it, and write a thank you note when I'm writing everyone else's?

TIA!

ETA: Our wedding is in five weeks.

Avion22

Cash it now, write a note now, but don't spend the money until after the wedding (if for some reason the wedding doesn't happen as planned, gifts, including money, are supposed to be returned).  

achiduck

You should always thank the gift giver when the gift is received, regardless of if the gift is cash or a material present. 

BinxRose

Awesome, thank you!