I knew it had to do with coinciding with the Little League World Series, but I had to go look up the details.
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"We expect to see a lot of smiles and style during Players' Weekend, which is underway in earnest in all games through Sunday. This is the game's embrace of individuality, charisma and creativity, a time when the colors are splashier, the uniform rules are looser and the appreciation for the path from youth leagues to the big leagues is made more apparent."
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What is Players' Weekend?
It's an opportunity for fans to get a little wider window into the personalities of the players. Established in conjunction between MLB and the MLB Players Association, all 30 clubs will be participating through all games Friday through Sunday, and every team will be in action each of those three days.
All clubs will be wearing non-traditional alternate uniforms, and each player is allowed to wear his nickname of choice on the back of his jersey, with a patch on his sleeve to pay tribute to a person or persons who aided their career."
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Why are players wearing colorful uniforms?
The vibrant, non-traditional alternate uniforms, designed by Majestic, were inspired by uniforms you would typically see in Little League, tying into the theme of the youth involvement that Commissioner Rob Manfred has invested in since taking office ahead of the 2015 season. Players are also allowed to wear and use uniquely colored and designed spikes, batting gloves, wristbands, compression sleeves, catcher's masks, and bats.
New Era is also providing specially designed hats, and Stance is providing the colorful socks."
http://www.mlb.com/news/2018-players-wee...-289570882
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And a compilation of all the nicknames:
http://www.mlb.com/news/2018-mlb-players...-289419008