What we did this semester is have Exec. Board sit down and plan out our entire schedule with advisors and alums, plotting out the new member program first and then choosing tentative dates for social events (i.e., ok, Tuesday the 13th is open for a dated/mixer, we'll talk to some fraternities; and Friday the 6th is the tentative date for Crush, etc). We typed it all up and distributed calenders to the chapter at the first meeting of the semester so people who work and stuff could kind of plan ahead for possible social events, not just have a week to try and find someone to cover for them, etc.
Anyways, back to your point. We had a spontaneous social event this semester- but it fell on one of the "tentative" dates, so lots of people were aware that something would likely be going on that night. We actually took out an ad in the school paper:
ALPHA CHI'S
FIND A DATE AND MEET AT _________
AT 8 PM
something like that. It was tons of fun and everyone had a blast. My suggestion is get a tentative date from this fraternity and let you sisters know that it's quite likely there will be a mystery social event that evening. That way they have a heads up, and hopefully it will work out
good luck
Maya