Reservation windows detail the time period when you can begin making reservations for your timeshare. Reservation windows are very important since prime weeks and resorts can go very fast.
In order to maximize your timeshare, you need to plan as far in advance as possible. The reservation window will detail when you have the ability to make a request or an actual reservation.
As discussed in the Hilton guide, Hilton provides various reservation windows depending on your status with Hilton. Hilton provides its higher tier members with greater reservation windows so that the can have the first opportunity to reserve their desired weeks.
Hyatt has various reservation windows that is discussed in the Hyatt guide. Basically, you will have six months from the start of your specific week to decide whether to use your specific week for the next year. You will also have 12 months from the start of the week to make reservations within Hyatt's internal system.
Each program has differences in their reservation window. It is important to understand these reservation windows in order to be able to begin planning the moment that the window opens. This will give you the most flexibility and change to get desired weeks.