Just what it says. No one is allowed to take vacation days during a tournament. Done to eliminate "vacation abuse". Tournaments can sometimes be held up for weeks or months by folks who want to "spend their vacation days" on a regular basis. Well actually it's not abuse unless you're deliberately stalling. Link? Vacation abuse:
https://support.chess.com/customer/portal/articles/1444860
What does "no vacation" in tornamants mean?