Based on the rest of our itinerary, the day it would work best for us to day-trip to Tangier from Tarifa happens to fall on a Friday. Rick's guidebook for Spain notes, "Friday is the Muslim day of rest, when most of the country (except for Tangier) closes down." How much is this likely to affect our experience? Will any sights be closed? Will it be difficult to arrange a guide?
Guides work every day, no problem. Some shopkeepers in the market may close for a few hours around noon to attend the mosque, as do staff at the American Legation Museum and Kasbah Museum, but not usually all day. The main sight in Tangier really is getting lost in the medina/old town, which is always open. I'm sure you'll still find lot to see on a Friday.