The ordinary modern calendar is designed to keep in synchronisation with tropical year rather than the sidereal year; this is so that, for example, the northern summer solstice remains around June 21 and does not slowly drift further and further away from that date.
The ordinary modern calendar (the Gregorian calendar) is designed to keep in synchronisation with tropical year rather than the sidereal year--in particular, in the way in which leap years are designated. This is so that, for example, the northern summer solstice remains around June 21 and does not slowly drift further and further away from that date.