Once you have configured your standard rate, you can overwrite it and set different values for certain periods.
Do this in Property > Rates and Availability.
Click on the price of the chosen date to change the single-day rate or on Long Term Definition > Rates to make mass changes to multiple room types and dates at once. On this screen you can also, if you want, set different prices for each day of the week.
Adjusting single-day rate
Adjusting rates by period and in multiple room types
Always remember that changes will only be synchronized with the channel manager by clicking Synchronize Rates and Availability. Set the interval to be synchronized, and click Synchronize rates and availability with the channel manager.
Learn more about rate synchronization here