Upgraded to Gold?
 

I went to checkin to a Marriot property and they informed me I was now a gold member. I checked online and it is reflected in my account. I was a Silver last year and have less then 20 nights for the year. Any idea what happend?

Rolling 12 month qualification year...

If most of your 20 nights were within the last three months, my guess would be the challenge promo. ^^ Did you also get stay credit for having a credit card?