God laid down this law, saying: if you want some good, get it from yourself.
—Epictetus, Discourses, 1.29.4
Here is how to guarantee you have a good day: do good things.
Any other source of joy is outside your control or is nonrenewable. But this one is all you, all the time, and unending. It is the ultimate form of self-reliance.
* Source: The Daily Stoic by Ryan Holiday and Stephen Hanselman