Apple watch does not hold a charge for a day

Apple's specs say that the watch should hold a charge for up to 18 hours. However, if your watch is older and has a lower battery capacity, it may not hold a charge for that long. Also, certain activities (lots of workouts, for example) will decrease your battery life more quickly.

To check your battery health, on your watch, go to Settings>Battery>Battery Health. If it's below 80%, you need a new battery.

This may help you see how varied the circumstances and how prevalent the problem is

https://discussions.apple.com/thread/253651514

There is a lot of this on the forum.

Has come up coinciding with software updates.

What Watch and what update are you on ?