There are two ways to make Rates available for a specific time period.1) if
If it's a Package Rate (which requires the Packaging Module: http://www.bookingcenter.com/products/modules/packages.html) then there is a 'from:' and 'to:' date field for each Package that would make a Package Rate available for only a specific time period.
2)
If it's a normal Rate (i.e., non-Package rate), then using the "Closed to Arrival" for a Rate can set the time period before and after your desired dates to not allow an arrival, so that it will only display for the dates desired. Thus, if you close the rate for all dates before June 1 and after July 31, then it is effectively available only for June 1 - July 31.
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Managing Rates and Availability to your Front Desk, Website, GDS and Global Booking Networks via MyPMS