Loyalty Traveler has the full low-down, in great detail, of Marriott’s July to October promotion. This offers triple airline miles, starting with your 2nd stay during the promotion period. For the Marriott brand, which is the one you are most likely to use if you are UK based, that means 6 Avios (or whichever other airline currency you choose) per $1 charged to your bill. This is not a bad deal at all if you are doing the stays anyway.
You will only see this promotion on your Marriott account if your earnings preference is set to Miles. You MUST register for it on the Marriott website – it is not automatic and you will not receive triple miles if you don’t.
If you elect to collect Points, you will see a different promotion. If you are an infrequent Marriott guest, you will probably be offered a free night at a Category 1-4 property for every 2 stays between 1 June and 31 August, with a maximum of three free nights to be earned. Registration for the free nights promotion closes 30th June so if you would prefer this then move quickly to sign up.
Your free night certificates are valid for 1 year, but Category 1-4 chops out most of the more attractive Marriott properties, so think carefully about whether you will use them or not. (More active Marriott guests will receive a promo offering x bonus points for staying y nights or in z different brands. Some people have reported success in being able to switch their targetted promotion by ringing Marriott Rewards, but this is not guaranteed.)
As Loyalty Traveller says, you can register for both and change your earnings preference for each stay, so a very cheap one-nighter can count towards a free night whilst an expensive five-night stay can be taken as triple miles.
(Full details of all current Marriott promotions can be found on our ‘Hotel Promos‘ page.)
If you need a hotel before July 22nd, though, I would strongly recommend my post on the Radisson / Park Inn promotion instead, which is effectively ‘buy one, get one free’ – with the benefit that the free one can be in a hugely expensive property whilst the paid one can be in a cheap one!