Though you may think it’s a good idea to congratulate a bride-to-be on her recent engagement, in actuality it is considered improper. “Congratulations” to the bride implies that she is to be congratulated for being lucky enough to land a man, when the luck is more properly assigned to the groom-to-be for getting such a wonderful fiancée.
The proper way to express your encouragement or happiness to the bride-to-be is to say “best wishes.” This lets her know you want her married life to be happy and fulfilling. It is appropriate to say “congratulations” to the groom, as he was the lucky one who got the bride to accept his proposal.
This tradition dates back to the Victorian era but has since fallen out of the realm of common knowledge. You might have caught a reference to the “congratulations” faux pas on the Season 4 finale of “Mad Men,” when Don Draper announces his engagement to Megan at his office and Pete Campbell corrects a co-worker, “You don’t say ‘congratulations’ to the bride — you say ‘best wishes.’ Congratulations, Don!”
A good rule of thumb is to keep your well-wishes consistent between the bride and groom: If you say “best wishes” to the bride, then make sure you say it to the groom, too. Oh, and avoid following that with “finally.”