Happy Birthday, Bridge!

Let’s, uh, brown bag a toast:

The Brooklyn Bridge was the toast of the town last night, but Mayor Bloomberg was one of just a few allowed a drink in its honor — and even that turned out to be illegal.

Bloomberg . . . drifted away from a closed reception and into Brooklyn Bridge Park — in violation of the city’s open-container laws.

“Is that wine in your glass, Mr. Mayor?” a Post photographer asked Bloomberg.

“Yes, it is,” he admitted. Bloomberg’s spokesman, Stu Loeser, later said, “It was an inadvertent mistake.”