Since you are posting in the Marriott Rewards forum I will assume you are looking for awards using your Marriott Reward points.

For popular hotels you need to book way out. Exactly how far depends on the hotel. For example for Rome, London and Paris hotels I would recommend 11 months. For places like Vail Marriott, they just released some award inventory for Sunday-Thursday night stays for the very end of March though mid-April (spring skiing). In many cases you must book 5 nights. For example you can book a standard award for April 5-10, but not for April 5-9 or April 6-10 or any shorter stays. They will release more ski season inventory sometime between now and the end of August, as their blackout dates are finalized then. Right now you can't even book a paid weekend stay for end of March/early April. For example attempting to book a paid stay for April 3-5 yields zip, even if you are Plat. As a Plat you can book April 3-10 and then call up and have them change it to April 3-5 under the Plat Guarantee.

You can also book Almost Anytime Awards for double the points at any hotel that has standard rooms available; except at the Rome Flora or Capri Marriotts who do not fully participate in Marriott Rewards. The Rome Flora and Capri Marriotts do have award availability but it is very limited and as such, very difficult to book unless you do so very far in advance.