Budgeting and Blitz Day (not quite sure where this one goes)

After many unsuccessful attempts to configure a set up I like for a computer I do plan to build, I've had to scrap the plans and begin again from scratch because of budget. I would have a computer setup that I'd like (case and everything in it) and then I'd forget about the key things that I'd need to use the computer (i.e. monitor, keyboard, mouse and speakers). I keep forgetting to factor in those. Recently (like a month ago) someone suggested that I buy all my parts on Blitz Day to save money. One thing has bothered me: How much is the sale? Is the savings large enough to warrant not even planning until that day and look everything up? Or, should I plan everything and then adjust for how much I go over budget? This problem is the main reason I keep scrapping my plans. Any help or advice would be greatly appreciated.