Details were made available on Flyertalk, but registration won’t open until May 1.

The skinny is this — One free weekend night for every three stays between May 1 and July 31.

These nights will need to be consumed by 19 December 2010 at properties up through Category 6 (so excludes Category 7 properties).

Hyatt’s offer is a free night every two stays, valid all week long, but with a shorter redemption period. Hilton’s offer is a free night every four stays (or 10 nights) with a slightly longer redemption period.

So this offer hits a sweet spot in terms of how easy or hard it is to earn a free night. And the free nights are valid for a reasonable period of time. But they’re only redeemable Fridays, Saturdays, and Sundays.

This is a good defensive move since both Hyatt and Hilton have free night offers on top of the usual points earning, and of course to my mind Starwood has plenty more properties one actually wants to redeem at compared to Hilton.

Whether this is an especially lucrative offer though depends on how your redemption patterns work, for most folks a weekend restriction is not problematic because vacation stays will correspond with at least a weekend and that’s how typical redemption patterns work. But if you’re not a weekend redemption type, the offer is far less valuable.

I do rather like Hyatt’s model of offering bonus points as an option. The free night is much more valuable, but there are people traveling so much for business without personal time to redeem the nights they earn, and such high volume customers would certainly value an option without near-term expiration.

But on the whole this is a very nice promo. At three stays per night it’s hard to get the economics to be overly favorable mattress-running jus for high-value free night redemptions, though it’s certainly not impossible. But it’s a big incentive to stay loyal to Starwood. And an even bigger incentive to hotel hop when you need to stay anyway. There are plenty of great clusters of Starwood properties, for instance I would probably switch back and forth between the Sheraton Grande and Westin in Bangkok because moving would just be so darned convenient.

I’ll no doubt offer up a reminder of this offer when registration opens.