Like any marketing campaign, you need to point out the benefits the venue receives by allowing you to use it for your speed dating event.
This should ideally occur between one month and two weeks before the event, allowing your readers long enough to plan for it but not long enough to forget about it. Announcing the event electronically is (generally speaking) free and very efficient, making a large group of people aware of it at a keystroke.
Disney Princesses can never be sure about the boys!
That's why they have decided to ask some questions to them!
(You should encourage others in your organization to do likewise).
You can also create an appropriate listing on Craigslist and post to any other social networking websites you belong to.