The 4 main Subbats were not created by man. They were believed to be here at the beginning of Earth. There was the Summer Solstice, the Winter Solstice, the Vernal Equanox, and Autumnal Equanox.
Summer Solstice: The time of the year to mark the beginning of summer, it is also the longest day of the year and the shortist night. The Summer Solstice is marked every June 21.
Winter Solstice: The time of the year to mark the beginning of Winter. It's also the shortest day of the year, and the longest night. The Winter Solstice is marked on December 21.
The Vernal Equanox: The time of year that marks the beginning of Spring. It's when the days are about even. This day happens about March 20 in the northern Hemisphere and September 22 in the southern Hemisphere.
The Autumnal Exquanox: The time of year that marks the beginning of Fall. It's the time of year that night and day are exactly even. This time of the year happens on September 22 in the northern hemosphere, and March 20th in the southern hemosphere.
These are the 4 Sabbats. You can ask about them to about anyone.