Throughout my career I've found Thursday is consistently "the day" when work people are most likely to go out together. In some places it may have been because the last Thursday of the month is payday, but largely I believe it's because the majority of people are likely to have commitments with other friends, partners, children etc on a Friday.

I'm also in favour of Thursday as the mandatory day if I need to be in the office as the last day of the week to do release management, e.g. minimise the risk of a bad deployment on a Friday affecting someone's weekend.