The green bagels are just plain Bruegger's bagels with green food coloring mixed in and were first introduced in 1997 at a Pittsburgh Bruegger's bagels. They've been an annual tradition at the chain for almost 20 years.
If you're signed up to their eClub, they're also sending a coupon good for $2 off a Big Bagel Bundle at participating locations. The Big Bagel Bundle includes 13 bagels of any variety plus two tubs of cream cheese.