It is appropriate to fly the flag every day. This list of days is declared in the U.S.Flag Code. The highlighted days are more common to fly a seasonal flag. 

New Year's Day - January 1

Martin Luther King Day - Third Monday in January

Inauguration Day - January 20

Lincoln's Birthday - February 12

Washington's Birthday - February 22

Presidents' Day - Third Monday in February

Easter Sunday- Movable- April 24, 2011 

                    or after the March equinox)     

Mother's Day - Second Sunday in May

Peace Officers Memorial Day (half-staff) - May 15

Armed Forces Day - Third Saturday in May

Memorial Day (half-staff until noon) - Last Monday in May

Flag Day - June 14

Army Day - June 14

Independence Day - July 4

Korean War Veterans Day (half-staff) - July 27

Labor Day - First Monday in September

Patriot Day - (half-staff from sunrise to sundown)       

Election Days

and such other days as may be proclaimed by the President of the United States; the birthdays of States (date of admission); and on State holidays.