Commas set off the elements of an address except for zip codes.

• Write to Bureau of the Census, Washington, DC 20233.
  
• In Needles, California, their luck ran out.
  

When a city name and state (province, country, etc.) name are used together to modify a noun that follows, the second comma may be omitted but is more often retained.

• We visited their Enid, Oklahoma plant.

but more commonly

• We visited their Enid, Oklahoma, plant.
  

Commas set off the year in a full date.

• On July 26, 1992, the court issued its opinion.
  
• Construction for the project began on April 30, 1995.
  

When only the month and year are given, the first comma is usually omitted.

• In December 1903, the Wright brothers finally succeeded in keeping an airplane aloft for a few seconds.
  
• October 1929 brought an end to all that.