I should also add—If it is an Orthodox event, you should wear a top that has sleeves, whether it be for the service or the reception. If it is a Conservative or Reform ceremony and you want to wear something sleeveless to the party portion of the event, you may want to consider wearing a cardigan, shawl, or shrug over it to cover your shoulders while you are in the synagogue for the service. I am not too familiar with codes of dress in Reform settings, but I imagine that sleeveless tops/dresses may be more acceptable there, however.