to celebrate an official appointment or to congratulate each other on a promotion, graduation, or success

那帮投机分子听说同伙被提拔了，个个弹冠相庆。

to flick dust off a cap, literally representing preparation for an official appointment following a friend's success

“弹冠相庆”的原意是弹去帽子上的灰尘，准备出来做官。