I don't know the specifics of the MHMC hotel arrangements, but the reason why Sunday shows are more affordable for a club is often the cost of renting a banquet hall on a Saturday. Wedding receptions are big business at most major hotels (they include not only banquet room rental, but also the VERY lucrative "dinner for hundreds"), and if you want to rent a hall on Saturday at most of the decent hotels they tend to be more costly - maybe beyond the budget of many societies.
As Sundays are usually relatively low demand days for large banquet halls, these rooms are more affordable.
We went thru this at SCAHMS when we got started in the 1980's, and had our show on a Sunday only for the first 7-8 years as a result.
Bill