you can enter then extend the visa exemption stamp at your local immigration office for a cost of 1,900THB, each immigration office has their own criteria on when you can extend it. some don't allow you to extend until a certain day count to expiration others allow you to extend It the day you land....
for land border crossings you get 2 per calendar year 01 JAN - 31 DEC.....
airlines are the ones that care about RT tickets due to the economics of it if you are denied entrance
You can either pay the overstay fine ฿500 per day…..or….go to your local immigration office and pay ฿1,900 for a 30 day extension…if in Bangkok you can do it the save day as you land as long as you land when immigration is open and you can make it there before it closes…good luck
Really you just learned that a government office is closed on a weekend, wow that’s just amazing but then again I guess your government offices in your country are open on the weekends….you can do it on Monday they might get you for an overstay charge of ฿500 per day….don’t get caught by the BiB over the weekend