Nowruz marks the first day of spring and the beginning of the Iranian year. It is celebrated on the day of the astronomical vernal equinox (the start of spring in the northern hemisphere), which usually occurs either on March 20 on March 21.

Equinox                Sunday, March 20, 2011, 23:20:45 UTC
Tehran                 Monday, March 21, 2011, 02:50:45 AM

San Francisco       Sunday, March 20, 2011, 04:20:45 PM*
London                 Sunday, March 20, 2011, 11:20:45 PM
Hong Kong            Monday, March 21, 2011, 07:20:45 AM

Sun= Sunday, March 20, 2011
Mon= Monday, March 21, 2011
*= Daylight Saving Time (DST)
.