Here in the U.S., most airlines offer last minute e-fares--typically available on Tuesday morning, for certain flights leaving late Friday night or, more likely, Saturday morning (when there are few business travelers) and returning the following Monday or Tuesday. In my opinion, many of the prices aren't really that good of a deal, especially considering what a specific timeframe you're forced into traveling.
Before their merger with CO
for a while offered Saturday-only same-day return e-fares. I was only able to take advantage once--flew IAD
on a 757 in the morning (with an upgrade to F, courtesy of the recently announced new policy of unlimited upgrades for Premier passengers), rented a car and drove up to Providence, hung out, and flew back to IAD
that evening. A fun little day trip!
I think we'll see fewer and fewer of these fares as airlines, at least here in the States, are getting much better at early on pro-actively cancelling specific flights on certain days of the week that are not booking well rather than scrambling at the last-minute to fill them with discounted fares.
"We hope you've enjoyed flying with us as much as we've enjoyed taking you for a ride."