The week is honestly just an illusion.
By that same token, time is just an idea that we made to make ourselves more comfortable with how everything progresses.
And, continuing that train of thought, a week is just a glorified chunk of time that humans established to make things more organized.
So, when we made the week start on Sunday and end on Saturday, and we made the original work week Monday through Friday, it is only logical that we look forward to Fridays because it represents a broader end of a week of work and the start of the weekend. Just imagine if we made the week start on Monday (so that Monday would be the new Sunday). The days would switch around and we would then be looking forward to a whole new day, a whole new scenario. What if Monday was Tuesday? Tuesday would become the new Sunday and we look forward to a newly created Friday, only to come back to work on what was the original start of the week which was Monday.
Or something like that.
All in all Fridays are pretty cool I guess.