Is the Nong Khai border immigration open on the weekend of August 30-31 for a visa extension?

You have a valid one year, multi-entry Non-OA visa (that you got from the thai consulate in your country before you came here) and that visa is still valid 🙂

AND

You want to bounce out/back before the Non-OA visa itself expires and get a new entry stamp for a year <- is that correct?

Remember you also need a new year of the mandatory 100K USD health insurance too (they will want to see it at the border when you stamp back in)

You're not talking about Immigrations either, you're talking about the border crossing people at passport control and those don't close for weekends or holidays, they're open every day of the year at the Nong Khai border .