It just depends on the parts. If it's a budget pre built rig or laptop then you'll probably see some sales around back to school. Components, usually around black fri, black november or black 3rd quarter whatever they've extended it to now. Some in demand name brand items don't go on sale often and when they do the sales are limited. For instance, intel cpus hardly go on sale for long. They stay pretty steady at near full retail, even when new models come out. No incentive for them to lower prices.
Graphics cards a lot of times will drop in price when an older 'enthusiast' model drops to 'mid grade' as new enthusiast models are released.
Other things can influence price. For awhile there was a huge excess of ddr3 ram a couple years ago and it was at an all time low. It's since gone back up (unfortunately). When a storm wipes out a manufacturing plant in asia like what happened with the seagate plant and wd plant a few years back, hard drive prices rocket. Many ati radeon video cards have bulked up in price with increased demand once people started buying them up for bitcoin mining.
If you go to pcpartpicker.com and search out whatever component(s) you're interested in, you can go to the bottom of the system build page and they show a chart with pricing trends on those parts. It may not be totally accurate, I'm not sure how correct it is but at least it will give a basic idea.