Pricing periods are defined periods of time within your registration period that generally have different pricing levels.
(eg. EarlyBird, Standard)
It is preferable to set the dates for pricing on this tab rather than on the Event Category tabs.
So if you wish to set three prices for the one category based on the date of entry then you could setup the pricing periods as above.
Then on the Event Prices tab you fill out the 3 prices according to date.