Santa Fe is high desert, so expect cooler temps at night. The average high in August is 83 and low is 53. Afternoon rain showers are not uncommon in late summer.
Santa Fe is 7,199 feet above sea level, and although you will probably feel fine, mild altitude sickness is something to be aware of. Please take care to stay hydrated before and throughout your trip and take it easy--you'll be on vacation, after all! I have included info in the "Things to Do" section about an oxygen bar in downtown Santa Fe that can alleviate symptoms, and please see more tips to prepare yourself here: https://bit.ly/2WirbYP
It's all about layering! Be prepared for cool evenings and consider bringing a light rain jacket. If you plan on walking from the ceremony to restaurant, bring shoes that are comfortable for a 15 minute walk. Other than that, swimsuit, sunscreen, and a hat might be smart.