Of course, they will still extend you. It would usually incur a one day overstay fine of 500 baht, but in most if not all immigration offices, they will not fine you on a Monday, if your visa finished on the weekend.
Go, you'll be fine (trust me I've made countless visa extensions over the last 12 years, more than a few on a slight overstay)
I thought it was great when they legalised it (even though I don't smoke). Then the shops started opening everywhere, and I remembered how cringe stoner culture it!