Tracy,
I'm confused... given your proposed itinerary above, are you trying to figure out if you should reverse your flights?
Fly into Rome> travel north > fly out of Venice???
IMO, I think your proposed itinerary is ideal...however,
here is Roberto's reply to a similar post:
"Usually people are advised to return home from Rome because flights back to North America depart in the morning and Rome airport is close to the city center (30 min by taxi). Both Venice and Milan airport are a distance from the center and that results on having to get up extremely early in the morning or find a hotel at the airport on the last night before the day of departure. If not for that, it doesn't make any difference to go from North to South or viceversa"
Regarding the national holiday, it is celebrated all over Italy, not just in Rome!
Hope this helps!