@Marc *****
you can go on the 20th but it might be best to go before in case you run into issues. Almost certainly (if approved) you’ll get a stamp from the day you apply, not added on to your existing stamp so you could lose a couple of days. Be aware that immigration offices around the country will be manic on Monday as they’ve been closed for 5 days. Lots and lots of people will be heading to an office then.