The organisation of the centres is overseen by the Syndicat mixte départemental des déchets de la Dordogne (SMD3) and they have a detailed page about the opening times.
You can narrow down your search by using the map towards the bottom of the page and finding the déchèterie closest to you, this then provides information on dates and times each centre is open.
SMD3 also say that all centres are closed on Sundays and jours fériés.
The summer opening times for the déchèteries especially on Saturdays are often based around odd and even weeks of July and August, or impaires and paires with the gates open on alternate weeks.
For example, if I take a look at the opening times for Saint-Cernin-de-l’Herm the summer service for the déchèterie between 1 July and 31 August is open on Tuesdays from 7h30 to 13h.
And then it is open on Saturdays but only covering the odd weeks for the two months and again from 7h30 to 13h.
Finally to add a little more spice to the service, a card the size of a credit card has to be applied for through the SMD3 that lets you open a barrier at the entrance of the déchèteries and entitles you to a certain number of visits each year.
Not surprisingly local opposition to both this system for the déchèteries and the overall cost of recycling and refuse services in the département is pretty strong.