It's science.Rule of Furnaces, they only start acting up just before a weekend or on a weekend. 100% of the time, iron clad rule.
I know the last time we had furnace issues it was on a Friday and we miraculously got a guy out to look at it after quitting time on Friday. He chased down a rare part for us that evening and got us back up and running at about 8 or 9 pm that evening.