Friday, for many reasons.
1) will motivate people to call in sick at work
2) --------------------------------------------- at school
3) go home early or never leave it
4) increase local delivery and take out restaurant economy
5) give the older a planned drinkin night
6) younger will play more videogames
7) the race to saturday morning would be accessible to almost everyone
8) in other time zones, it's even better for them
9) i don't work fridays
10) is a source of calcium and is recommended by physicist
11) Would be easier to find names that fit. Like "funky fridays" or "filthy fridays" or even "fridays fridays"! Who knows how far our imagination can go.
12) Payday is thursday for most people. Can donate more at this time
$$
and so on. I could go at it for a while.