So the App I would design would help keep track what you have already planned and help keep track of your budget.
This would be the opening screen. As you can see you just three tabs. The twitter one would be for you to share whatever you have planned and to keep your guest posted on what is going on if you would like too. Other then that the Wedding Planning tab lets you add what you have already accomplished and shows you what your still lacking. The budget app lets you keep track of what you spent on each item and if you have gone over budget or are under budget.
Here is the example of what the budget tab looks like. If it were real you would be able enter to enter the amount you have spent on each item and it would give you the total. Then under the total it would tell you your budget and if you were over or under.
If the app was real I am sure brides would use it because of how handy it is to use. It is convenient and easy to use.