May in Phuket is very hot, but there can be rain showers in the afternoon/evening, so we suggest bringing a light rain jacket. The sun is VERY strong and it's very easy to burn without realizing it (trust us from personal experience), so we do suggest lathering up with sunscreen before going out in the sun - especially prior to the celebrations :) Fortunately, there is always a pool, ocean or cool drink nearby.
You should be able to navigate relatively easily if you only speak English. The island sees thousands of tourists year round! That said, it's great to learn some common phrases prior to your arrival, like 'Hello!' is Sawasdee ka (Sah Wah Dee Kah). Easily enough, the same word for hello, is also the same word for goodbye. 'Thank you' is khàawp-khun (kahp-kuhn). And 'the bride is beautiful' is: เจ้าสาวสวยจัง. Extra points for repeated use of the latter.
A visa is not needed for citizens of the US or China, most European countries and the UK if you are staying less than 30 days. You can double check the requirements here if you are unsure: https://visaguide.world/asia/thailand-visa/who-needs-a-visa-for-thailand/ Passports are a necessity! If you do not have a passport, we suggest getting this ASAP as it can take 6 months or longer to process. You must also have 6 months left of validity in your passport so double check your expiration date before you book your flights!