My top 3 tips on picking offer expire dates:
1. Pick a day in the middle of the week, especially if you have an email or fax list to the same people so you can broadcast a 24-hour's left alert the day before to bump response.
2. Never pick a day near the start of a month (such as Oct 1st) because everybody the month before thinks the next month is much much further away than it really is. Way more urgency to Sept 30th than Oct 1st.
3. In B2B marketing (especially events with a longer lead time) always tell them how many business days they have left because that's much more urgent than regular days. Example:
You have just 3 business days left to save $100.00 on your ticket to the "Email Newsletter Publisher's Profit Workshop" at http://sherpastore.com/store/page.cfm/1979
(Um, yeah that was a shameless self-promotional plug there at the end, but hey, I gave value.)