It's fair to say that the main purposes of calendars have to do with scheduling future events and with coordinating human activity. Although the first day of Spring, Summer, Fall, or Winter may coincide with certain astronomical events and may be assigned specific dates on a calendar these are only points of reference. It may be that certain flowers predictably bloom in specific locations within a certain range of dates, but there is some variance from year to year. Mostly this is due to weather patterns which can vary greatly from year to year. For instance, where I live sometimes the last hard frost during spring might occur in late April, it could just as easily occur during early summer. Nature doesn't seem to really pay attention to the human invention known as the calendar!