Well, since one place you said cheapest day to buy and the other looks like the cheapest day to fly, I will give you both.
For the most part, all airlines run specials on Tuesdays and Thursdays with lower rates than the rest of the week. The big thing is to avoid weekeneds when it will go up quite a bit. But sometime mid-week is good. Again, for most sites it is on either Tuesday and/or Thursday.
As far as when to fly, this does vary a littl based on where you will be going. But for the most part, you again want to try and travel mid-week. Tuesday, Wednesday and Thursday are usually the lowest. Monday's and Friday's are popular days for travel because people use them at the beginning or end of their trip to get an extra day. Finally, if you don't have restrictions on time, try to go later in the day, around the after noon or early evening. This will usually get you lower fares. Sometimes, late evening on Saturday is also good. You have to remember, most people want to fly in early morning and fly home late at night to get the most out of vacation, or for business people. So, if you can take advantage of these slow times, you will be able to get slightly lower fares.
Hope this helps!