I’m surprised the illustrious genius of Cecil Adams did not pick up on the rationale behind the superstition of eating cherries (or strawberries) and milk on a warm day. Before the days before Pasteurization and refrigeration, you had to drink your milk before it went sour, which meant you drank it in the morning. On a warm day, it would be bad by lunchtime.
Cows are milked early in the morning. Fruit, such as cherries or strawberries, is harvested during the day. If you consume the two on a warm day, by the time you have collected enough fruit for a meal the milk will be sour.
They had cool places that would keep milk fresh for several days.
Cherries and strawberries themselves keep for several days or weeks regardless of temperature, and no one would wait until after berries were picked on a particular day to eat them.
Sour milk typically isn’t harmful, and until pasteurization and refrigeration were common was consumed regularly by most people. We throw away regular milk that turns sour and turn around and buy yogurt and cottage cheese and buttermilk. We could just consume regular milk that soured in the fridge, but there is always the slight chance that something harmful grew in it, and it takes some technique with temperature and starter cultures to get the various specific flavor and texture profiles we like.