Use commas to properly punctuate dates, numbers, and places.
Example: He was born on August 3, 1999.
Example: The United States is a country of approximately 300,000,000 people.
Example: July 4, 1776, is known as the day America won independence.
Example: Washington, D.C., is the capital of the United States. Many people from Frederick, Maryland, and Alexandria, Virginia, commute into the city to work every day.
Note: The year, abbreviation, and state names in the above sentence above count as extra information (See Rule #4). Even if this information is critical to the meaning, we conventionally separate it from the rest of the sentence using commas specifically with dates and place names when two pieces of date or place information are used together. Without the year, the sentence would read: “July 4 is known as the day America won independence”, and without the date, the sentence would read: “1776 is known as the year America won independence.” These are both complete sentences.
The same goes for the following two sentences without the place modifiers: “Washington is the capital of the United States. Many people from Frederick and Alexandria commute into the city to work each day.” Also consider: “D.C. is the capital of the United States. Many people from Maryland and Virginia commute into the city to work each day.”