i just tried to do a dummy booking on ua220 for 12/18 and in addition to having the the little "x" in the selection circle so one cannot select the flight, the flight is all zero's which indicates that the flight is totally sold out but as Benny8444 says, you should check-in on-line and you do want to be checked in as soon as possible and not be one who tries to check in an hour before the flight. you still might not get an actual seat assignment but you should be able to check-in and get you departure management (dm) card and then a seat assignment at the gate (and personally, i would get to the gate early