My 30-day tourist visa will be up this Saturday. I presume that the immigration offices are not open on the weekend (although I can't find this information in English anywhere online), so I assume that it will be acceptable for me to extend my visa on the following Monday, and not be charged for this. If I really have to extend tomorrow then I can, but I would prefer to avoid this if possible. 

 

I'd be very grateful if someone could advise me on what I should do, thank you.

You will not be able to do the extension tomorrow since immigration will be closed for a holiday.

You could of done it a week ago without a problem and would not of lost any days since the extension starts from the end of your 30 day visa exempt entry (it is not a tourist visa).

If you apply for the extension on Monday you will not be charged for an overstay of 500 baht per day. If you don't do it on Monday you will be charged for every day of overstay.