Every bride likely has a big to-do list mapped out months in advance, but if you're the wedding coordinator, you'll need a to-do list of your own for the big day to keep you organized. Even if you've done a hundred weddings, these events can quickly devolve into chaos if you try to keep everything in your head. Having a basic list, will keep you in control on the day the bride needs to trust you most.
Check in With Bride and Bridal Party
When you first arrive at a wedding venue on the big day you'll want to check in with the bride and bridal party. Make sure it's clear they should come to you with problems and not the bride. Make this announcement in front of the bride so they understand the bride agrees. Many people don't understand that the purpose of the coordinator is to funnel all the problems and issues away from the bride so she can relax and enjoy her day.
Set up any decoration you're in charge of. Some wedding coordinators do flowers. Some do reception decor. Don't leave this until later or you'll get wrapped up in other things and it won't get done. Once people know you're on site, get this taken care of.
Check in with Vendors
Meet briefly with each wedding vendor. Introduce yourself to them and let them know where to set up and if they have any questions or problems that the bride has authorized you to field all those issues. Be as cooperative as possible when working with vendors you haven't worked with before. Sometimes vendors get stuck with heinous wedding coordinator control freaks and they may be on their guard until you've proven yourself to be sane.
Help the bride round up everyone for pictures. You will need to do this twice, once for the pre-wedding photos, and once for the after-wedding photos. Since most brides don't want to see the groom before the wedding, logistically some photos won't be able to be taken until afterward.
Make sure everyone involved in the ceremony has what they are supposed to have. Microphones should be checked; music should be confirmed. Check that everyone is in their place. When the music starts, direct the flower girl and bridesmaids on when to go in.
There may be some activities for you to oversee at the reception as well. Check on the caterer and make sure all food preparation issues are being taken care of. Help get the party started on the dance floor by making sure the DJ is playing the songs the bride wanted played.
After the wedding stay to help clean up. Make sure things like wedding cake toppers, food plates for the bride and groom to take with them and cake serving tools are all packed up for the couple. Help them load their gifts in their car.