M is 44. She was born in Los Angeles, her parents are from Guadalajara, Mexico. She told me this custom about St. John the Baptist day in person.
“El dia de San Juan… June 24th, you’re not supposed to go in the water, like not even take a shower. It’s a religious belief… I remember my mom always telling me I couldn’t even take a shower that day… I’m not sure why, I think because on this day water is holy?”
San Juan, or John the Baptist is associated with water because he baptized Jesus. Some observations of this day involve the opposite of what M told me, where people bathe and splash in bodies of water (see https://www.nativeseeds.org/blogs/blog-news/celebrate-dia-de-san-juan). For information about celebrations in Spain involving fire, water, and plants, see https://centromundolengua.com/the-night-of-saint-john-in-spain/.