ante diem vi idus augustas

I have nothing for this day, but I notice a pile of sites claim this as the beginning of the Panathenaia (I'm not sure how that reckoning works) and/or that today was some celebration in Rome in honour of Venus. There are a number of such festivals which I have no idea in regards to the source, but I suspect a 19th century 'pagan book of days' or something like that. If anyone can pin down the source of such things for me, it would be greatly appreciated.