Can US passport holders do a Mae Sai border run on a Sunday?

Yes, it's possible for US passport holders to do a Mae Sai border run on a Sunday, as borders are open on weekends and public holidays. However, it's important to note that there is a limit of two 30-day border runs using Visa Exempt Entries within one calendar year. If you have already done two runs earlier in the year, you won't be able to enter again without a visa.