They haven't announced what dates the bring a friend free coupons will be yet, we will find out on opening day when the coupon books are issued though.
Typically 1 is good on Mother's Day, Father's Day, any weekday in early June, a September day after labor day and 1 other day.
Sadly with the new management at Six Flags, there have been a lot of cutbacks so the bring a free coupons may not be as good next year. They also got rid of Funatics Rewards which allowed you to earn a free ticket for a friend good any day of the season plus a skip the line pass, free soda etc. They are also cutting the operating schedule. This past season the park opened on March 28th, for 2011 it is 2 weeks later April 9th and the Wiggles are gone from the park (thy didn't want to pay the licensing fee)
edit- here are the bring a friend free dates from 2008
Bring a Friend Free:
BAFF Mother's Day, May 11
BAFF any day Mon-Fri June 2-20
BAFF on Father's Day, June 15
BAFF Mon-Fri Jul 21 - Aug 1
BAFF Sep 14, 20, 21, 27, 28
BAFF to any OTHER Six Flags Park (this was not in the 2009 or 2010 book, one of the past 2 years had the same coupon but for half off admission instead of free).