From Anstepedia

To within reason, there are four IC years to every one OOC year. Years officially change on each equinox and solstice:

Vernal Equinox 
Approximately March 20th
Summer Solstice 
Approximately June 21st
Autumnal Equinox 
Approximately September 22nd
Winter Solstice 
Approximately December 21st

In some cases, Time may reach a standstill, depending on the current plot being run. The official year is always on the Main Page.