The Japanese calendar system was changed after Meiji restauration.
Originaly the lunar calendar, called wafu getsumei (和風月名) -（わふうげつめい) used Japanese names for each month.
Their names are adapted to the seasons and events of the lunar calendar, and are sometimes used in the current calendar as well, but they deviate from the current sense of the seasons by about one to two months.
The details of lunar calendar is given here, it is necessary to undersand old dating system :0