Chunfen (春分) is the vernal equinox which is on March 20-21. In 2015, it falls on March 21.
An equinox occurs twice a year, around March 20-21 and September 22-23. It is the day when daytime and night are of approximately equal duration.
The names vernal equinox and autumnal equinox are based on the seasons, and can be confusing since seasons of the northern hemisphere and southern hemisphere are opposites, and the vernal equinox of one hemisphere is the autumnal equinox of the other.
Another way to call them is March equinox and September equinox, referring to the times of the year when such equinoxes occur rather the seasons.
A few Chinese poems related to Chunfen (vernal equinox):
Further readings :