Here you can set which nights of the week this rate price applies. If it applies to all nights make sure all check boxes are ticked.
For example if you wish to define a weekend special rate price you could deselect Monday to Thursday and this rate price will then only be available on Friday, Saturday and Sunday nights.
If you deselect certain nights they will show as not available unless you have another rate price set that includes those nights.
This rate price will only be offered on the checked days.