Organizations often have "Missions Statements," "Vision Statements," and "Core Values" posted somewhere where their members, customers, or visitors can see them.
A mission statement describes what the organization is for; what its purpose is: "What are we doing?". The vision statement describes how the organization benefits the community it belongs to: "Why are we here?"
The core values offer a guide on how they are going to get where they want.
Oftentimes missions, visions, and values are just words to make sure that human resource or marketing boxes are checked.
When making today a good day, it's valuable to have a mission, vision, and values and to make sure that you are on the right track by checking them regularly.
A personal mission may be very specific or general: "I want to buy a thing of value," "I want to be a better friend," "I want to be a good cook," "I want to be more healthy." Why we want those things will inform our vision and who we are make up our core values.
When they aren't defined, it's difficult know know how we're doing day-to-day. It's possible to have multiple missions statements. It's possible for a mission to change over time.
Exercise: Write down a Mission Statement and a Vision Statement. Put it where you can see it. Ask yourself throughout the day/week/year if what you are doing is working towards that.