When you dig your potatoes depends on how big you want them. I like "new" potatoes the best so I start checking them after they have bloomed. You can carefully dig around to see how big and about how many you have per hill. I even pull out a few and cover the roots back up with soil and they will keep on producing.
If you want to wait untill you have large potatoes to keep through the winter it is best to wait untill the tops of the plants start dying which is usually in the Fall of the year.
I do it both ways, dig some early for new potatoes and leave some for keeping through the winter.
If you meant what sign as in the moon signs I have no idea,I've never bought into that.