Why does our porta potty smell like rotten eggs after a hot day?

That rotten egg smell is hydrogen sulfide gas, and it is not normal for a properly maintained unit. It means the deodorizer in the holding tank is not keeping up with the waste breakdown. Full sun on a hot day accelerates the bacterial activity that produces the gas. A standard weekly service visit is not enough for a unit in heavy use with high heat. Your unit is sitting for a week between services, and the heat is overwhelming the chemical treatment. The deodorizer may be a formaldehyde-free deodorizer, which is better for the environment but sometimes less aggressive at odor control in extreme conditions. For your wedding, you need to increase the service frequency. With 140 guests, food, and alcohol, one standard unit is at the lower end of the guideline. You should request a mid-week service before the event and a service the morning of the wedding. Also, ask the rental company if they can apply a stronger deodorizer concentrate to combat the heat.