no worries pal. Get down there soon though, it's 2pm already. Have a visa photo, 1900 baht, copy of your photo page, visa stamp page and tm30...bring a pen.
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)