If you saw a December 17 notification on the 16th, it could be a time zone difference. I'm just guessing. Aside from that, there's nothing unusual. You just do what the notification says and wait for the letter with your appointment. You wouldn't expect to receive it the day it was sent, so I'm a little perplexed by your question. You should get it soon and, if you don't by January 16th, follow the directions in the notification.