A friendly reminder email would work. Keep it light and easy, more along the lines of saying that you just wanted to share that the event will be taking place on XX date and is shaping up to be a great event. You are grateful that you, XX business, have decided to help support us!
What is accepted etiquette when it comes to contacting business donors after they have agreed to donate something? Our event is coming up and I'm getting a bit nervous that I haven't received the donations from them yet.