RE:help me understand...Well your post set-up the basher that followed quite well, like a straight man on the Jimmy Kimmel Show. Notwithstanding.. I'll answer your question as though you are a legitimate newbie investor for those whom may be watching and truly are.
Just par for the course hmb. If you look at the charts, Monday starts a downtrend in SP until "hump day" aka Wednesday and then the SP rises toward the weekend. If there's been a high percentage SP increase during the week, you'll see selling on Friday afternoon meaning the day/week traders are exiting their positions prior to the weekend. If the upward SP momentum trend remains strong and on Friday, generally speaking this is due to an expectation of a gap-up on Monday.
Watch the trends.. my friend.