https://techcrunch.com/2019/08/21/how-to-protect-and-clean-your-apple-card/

First of all, keep it out of the light. It hates bright light, especially sunlight, it’ll kill it. Second, don’t give it any water, not even to drink. But the most important rule, the rule you can never forget, no matter how much it cries, no matter how much it begs, never feed it after midnight.

Oh, and Apple says the card should also avoid contact with denim and leather as such materials “might cause permanent discoloration that will not wash off.”