A search will provide a handful of threads with not only the date but exact time that your upgrade window opens.

If you booked through SABRE, your upgrade will process at 1am EST on the day of your window, so in your case on the 20th. If booked in SHARES, it's still on the 20th but I'm not sure we've pinned down the exact time.

(If booked on the website, it's SHARES, if booked on Orbitz, etc it's SABRE, if booked via telephone, I think it could be either.)