And Wimbledon's website asserts it's all about the grass:
Wimbledon is the only Grand Slam that has a day free of play in the middle of the tournament.
Why? It has nothing to do with old-fashioned British decorum. Nor is it a hangover from the days when everything closed on Sundays and everyone, except emergency workers, had ‘the seventh day’ off to recharge. The All England Club is closed on Middle Sunday simply to give the courts a breather. It’s all about the grass.
“People often ask, ‘Why don’t you play on the Middle Sunday?’, but it’s not about having a day off,” confirms Philip Brook, Chairman of the All England Club.
“The main reason is that it gives the groundsmen the only opportunity to put a proper amount of water on the courts to get them ready for the second week.”
So while the players go into competitive 'pause mode', head groundsman Neil Stubley and his team of 28 are in at dawn to tend to the precious grass.