Procrastination
Sunday best is fine. This means you don't need to wear a suit and tie but also don't show up in flip flops and shorts.
The event you've been invited to is just the reception. We will be having a limited ceremony the day before, basically just our parents and best man and maid of honor.
Food is being catered so no need to bring anything in that department. Alcohol won't really be provided so if you want to bring a bottle of something feel free to.