never and right now. lol. There are always deals on, holidays or not, but they may not always be the parts you want. Depends how flexible you are or if you picked your parts and want those parts for various reason. I was torn, my bday was the start of the month, built myself a new PC for it. I was looking almost two months beforehand, and had it on PC part picker. Every week, new sales would come, and some would end. The price of the total system always seemed to stay about the same. If I would have bought the parts over the two months I was looking, compromised on parts I wanted for other brands that were on sale, etc, I could have gotten a better deal, but on the parts I wanted, same price over 2 months.
There are sales going on all throughout the year, but you should purchase all your components at once. You cannot guarantee that the components you're wanting will even be on sale during Black Friday/Cyber Monday. The best time to purchase your components is when you can afford all the components you're wanting.