I've planned or helped plan quite a few weddings: a couple for myself, a few for my family, and a few for my freinds. I've researched venues in NY/NJ, in Washington DC, in SF Bay Area, in LA, and in Atlanta. I'm not a paid planner... don't mean to imply that I am... but I'm a research nut and like to help people get a good deal.
Here's what I've found.
It is REALLY venue specific what discount, if any they offer you for Sundays and Fridays or for Holidays. In EVERY city I looked at I would call one place and they would say, "Labor Day Sunday is the same price as Saturday, same capacity," I would call a place RIGHT down the street from there and they would say, "Oh Labor Day Sunday, we can offer you 180 a person with a minimum of 100 instead of 210 a person with a minimum of 150. "
It made no sense... it's just property/management's perception on the popularity of that date. So call around! You will be surprised by what you find.
Another money saving tip for Friday or Sunrday weddings: Some places will make a deal with you if you coordinate your menu with another wedding's menu for that weekend-- this works with Caterers, Restuarants, and Hotels. If say Saturday wedding is servicing Halibut and NY Steak, and you are willing to have the same or very similar menu... they will work with you. Sometimes its not an outright discount, instead it's a "well, if you do the same menu instead of the Halibut and Chicken you were thinking about, I'll give you the NY steak at the same price as the chicken."
Still it's helpful.
Another thing I found is that some hotels like the Four Seasons and the Intercontinetal hotels are more willing to work with you than the W hotels or other 'modern' hotels.
With all the research I've done... IF we don't do a destination wedding, we will be doing a Holiday Sunday Brunch reception wedding at a place that offers a Sunday discount.
Some photographers, DJ's and videographers will also offer a slight discount for a Sunday or daytime wedding. It's usually not much... a few hundred off, but it's all helpful.
GOOD LUCK!