it’s because the day before the holiday weekend is a day when a lot of jobs let their employees go home early,like on christmas eve or the night before thanksgiving, or friday before a three day weekend (memorial day weekend, etc.). they can’t let the wall streeters go home early if the market is open, so they close it the whole day and they can either leave early or have the day off. so it’s like a little gift from the employer, the chance to have a little extra time off with pay.