In this instance you should honour your quote becuase you clearly knew 4th of July was a holiday when taking the booking.
For other holidays it's totally up to you. I received an inquiry about a NYE wedding this year and have quoted $250 extra, I would like to spend NYE with my girlfriend but it's not a huge deal. If however we had plans to actually do something good this year then my happy price might have been $2000 extra rather than $250.
Don't ever tell them no, tell them how much.
