Ever since Sakurai announced Super Smash Bros. 4, he began revealing returning characters at random times of the month and if he has not revealed anyone within a month, it is most likely that he is building up for a newcomer reveal (which will most likely occur on a Monday).
But when Sakurai started becoming quiet with characters announcements due to E3 2014 and other things, fans of the Xenoblade Chronicles series began building hype that Shulk will be revealed whenever a Monday is about to occur.
While this was quite obscure at earlier times, it became much more famous when any newcomers were announced for a reveal on July the 14th, a Monday, with Monado Monday becoming extremely popular on Miiverse along with having the previously unbroken Gematsu Leak confirming him.
To many fans' dismay, the reveal ended up revealing Robin and Lucina from Fire Emblem Awakening, which both revealed the Gematsu Leak to be a hoax due to Chrom being de-confirmed, another hyped character by fans, which frightened many people that Shulk would not be playable. However, Shulk was shown in the ESRB leak as a playable character, and on 8/29/14, Shulk was confirmed through a Japanese Nintendo Direct. However, this happened on a Friday, not a Monday. This then resulted with people renaming Shulk's reveal day as "Feelin' It Friday," a reference to one of Shulk's lines in the reveal.