My friends and O will be travelling in Paris for 15 days and in 6 days in Barcelona. We applied for our visa and got a 1 month visa. But we can only stay for 20 days on it. Even though we planned a 21 day stay. Will this be an issue? We leave in two days so we cannot even give it for extension. How should we proceed?
It seems the French consulate made a mistake or only considered the stay in France (that's 15 days + a 5-day grace period, for a 21-day stay, you would customarily get 30 days or more).
You must in any event abide by the maximum stay mentioned on your visa. If you leave later, you could be liable for a fine and could also have trouble getting another visa later on. For a single day, you might get lucky but that's still the rule.
Worse still, the border guards might notice that your return ticket is too far in the future when landing in Paris and deny entry entirely. It's also entirely possible they don't notice or just let it slip but that sounds risky.
Similar real situation : My brother did miss his flight back home from germany and it was his last stay day visa so after he changed the flight (for tomorrow) he was asked to do a later check-in (even though it was technically impossible) and by entering the internation area (transit area) he was considered "left in time"
Go to the departure airport on the last valid day of your visa. Try to check in - the airline staff will point out that you are a day early. Go to passport control with your expiring-today visa and your departing-tomorrow ticket and try to look embarrassed for not noticing the departure date. The Spanish are fairly relaxed people, they will probably allow you to "depart" and then stamp a 72-hour shore-leave visa in your passport.
They will be less relaxed if you overstay and they notice. Penalties could range from "don't do it again" to jail time.