Are you asking about adding weeding dates in FTB (family tree builder) or on the site?
If it is on FTB, then when you add a spouse to a person it shows the marriage link between them and another window appears below showing that they are married with the family’s edit button. From that edit button choose `Edit details` where you will find the option to enter marriage date also.
If it is on the family site then again when you add a spouse on the left hand side in facts part you see marriage as a fact. If you select it you will see the option to edit the fact where you can enter the marriage date.