This app was mentioned in 3 comments, with an average of 1.67 upvotes
Simple Moon Phase Widget - https://play.google.com/store/apps/details?id=com.SimpleMoonPhaseWidget&hl=en_US
Because I work with the public and it's true that the moon affects them. What they don't tell you is the new moon makes them crazier than the full.
There's a great widget/app for Android that does exactly what you're looking for. You don't specify a platform, but there's also one available for Linux.
It's the simple moon phase widget