So basically, as long as I get this postmarked by midnight of the 5th day (according to my contract) they should honor the cancellation?
We purchased Saturday 10/15 so we should have until midnight Wednesday 10/19 correct? I'm assuming that 10/15 actually counts as one day so we don't have until the 20th.
Postmarked is indeed the operative and important word here. Later date of actual receipt matters not.
If you meet the date deadline, there is no "should honor". Rescission (cancellation) rights are provided by applicable state law, not by the kindness or benevolence of the developer. They simply have no choice other than to process your rescission and issue 100% refund of any deposit paid (although it could conceivably and lawfully take up to 45 days before you actually have the refund in hand).