It doesnt only depend on the day of the week, but sometinmes you can cross over into another season, that can cause a fare to be more or less, this happens often with Intl fares. then also theres the time factor, the T&Cs of each fare is different so a stay of X amount of days might be more or less thats required in order to within the rules of a certain fare. if theres a 5 day max stay and you are doing 6 it will probably change. if theres a 4 day min and you want a 3 day stay it too will change.

then depending maybe to where you are flying and Time of day that too can cause a difference in fare.

On a recent purchase I wanted to go on a Mon but Tues was cheaper, when I went to book the same itin for another trip a few weeks later it was teh opposite and Tues was more then Mon. other time sit can simply be that teh particular day youre looking at already sold out in the cheaper or any particular fare type that you were looking for.