US anime cons are year long. Any month of the year you can find a con to go to. Remember that your summer is our winter.

Here's a listing of the cons by area and dates. So just decide what else you want to do while you're in the US and pick a con.
http://www.animecons.com/events/state.shtml/002
Here's a listing of the top 10 US anime cons in 2008.
1.
Anime Expo - Los Angeles, California: 43,000 estimated warm bodies
2.
Otakon - Baltimore, Maryland: 26,394 paid attendees
3.
New York Anime Festival - New York City: 18,399 warm bodies
4.
A-Kon - Dallas, Texas: 15,000 estimated paid attendees
5.
Sakura-Con - Seattle, Washington: 13,600 estimated paid attendees
6.
Anime Boston - Boston, Massachussetts: 14,339 warm bodies (13,248 paid attendees)
7.
Anime Central - Chicago, Illinois: 13,900 estimated warm bodies
8.
Anime North - Toronto, Ontario: 13,300 estimated warm bodies
9.
FanimeCon - San Jose, California: 13,000 estimated warm bodies
10.
Anime Weekend Atlanta - Atlanta, Georgia: 11,101 paid attendees