What day of the week is best time to check for domestic airfare deals?

I haven't flown for many years. I recall seeing a consumer story on TV in the past that certain days of the week are best for checking for low or lowered airfare prices. I'm only talking about domestic US flights. Anyone heard of this? Best days or time of day to check online for bargains/lower prices for airline tickets? I'm planning to fly in May, but want to shop for good deals now, as my budget is tight. Thanks!