The Wedding Day:

• Assist the bride getting into her dress, make-up and hair if needed.
• Ask the bride if she needs anything (food, water, etc.) all the way up until it is time for her to walk down the aisle.
• Assist the other bridesmaids on where to go and timing, especially if they show up late.
• Help make sure the bride has privacy if she needs some quiet time before the ceremony.
• If needed, assist guests to their seats and be social prior to the wedding. 
• Always be on the lookout for the unexpected problems and assist when needed. If you are close enough, fix what needs fixing so the bride, groom and their families can focus on the ceremony.
• Be social and dance at the reception.

Just like the maid of honor, your most important job will be to provide moral support and be a good listener. Keep in mind that planning a wedding is very stressful and you want to keep the bride calm and in a positive mood. It is a job to be taken seriously and will require a lot of work on your part.