You have to wait for a feast to be organized, when a feast is in progress and if you meet your husband to be if he's not at the feast yet he'll suggest that you go to the feast. Once both of you are at the feast both of you will take the vow.
In one case however, I'm not sure what the conditions are, the lord would say he can't wait no more and tell you to take the vow at once. Happened to me when I went to talk to my fiance lord at a castle after a few weeks of getting engaged. The vows were exchanged (outside of the castle, because I asked the guards to meet him instead of going into the hall, might be an oversight and shouldn't be possible) and we got married.