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The US Weather service usesThe US Weather service uses your definition... When they say, for example, this was the wettest spring they mean the calendar months March, April and May. The "first day of Spring" terminology came to be because Christians did not want to print the names of pagan holy days (Vernal Equinox...) on their calendars.

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?