Costco hours and holiday schedule for 2014:

Warehouse Hours
Monday – Friday 10:00 AM – 8:30 PM
Saturday 9:30 AM – 6:00 PM
Sunday 10:00 AM – 6:00 PM

Independence Day: Wednesday, July 4, 2012 Closed
Labor Day: Monday, September 3, 2012 Closed
Thanksgiving Day: Thursday, November 22, 2012 Closed
Christmas Day: Tuesday, December 25, 2012 Closed

Previous year(s):

Costco is open today Martin Luther King day.

Costco is closed 4 days out of the year. Although this info was gathered from the website, it is always advisable that you call your local Costco, to make sure.

You can see the dates and information here:

https://shop.costco.com/en/In-The-Warehouse/Holiday-Hours-and-Closures.aspx

Warehouse Hours
Monday – Friday 10:00 AM – 8:30 PM
Saturday 9:30 AM – 6:00 PM
Sunday 10:00 – 6:00 PM
New Year’s Eve: Friday, December 31, 2010 9:00 AM – 6:00 PM
New Year’s Day: Saturday, January 1, 2011 Closed
Easter: Sunday, April 24, 2011 Closed
Memorial Day: Monday, May 30, 2011 Closed
Independence Day: Monday, July 4, 2011 Closed

New Year’s Day
Easter
Memorial day
Independence Day

(strangely enough they did not mention anything about Christmas)
Source(s):
Costco Website