In the United States there are no legal or official definitions of the seasons. Nothing important depends on them, they are just rough divisions. The definition I was taught was that winter is the three months of December, January, February; spring is March, April, May; summer is June, July, August; fall is September, October, November. Meteorologists on TV have been putting forward the idea that the seasons should be considered to begin at the equinoxes and solstices, as explained by Jmat. I think that's silly myself. Very informally, we often think of the summer as beginning on Memorial Day and ending on Labor Day. The weather is so variable in the U.S. that the seasons don't really correlate very well with the weather. Within the past week, we've had temperatures here below 5ºC, which feels like winter, and today it was well over 20ªC and sunny, which felt like summer. The range of climate variation within the U.S. is huge, though probably not as big as in Russia.

There are two days every year when the amount of daytime and nighttime in the day are exactly equal. These are called 'equinoxes'. They mark the beginnings of Spring and Autumn. The longest day of the year (the summer solstice) marks the beginning of summer. The shortest day of the year (the winter solstice) marks the beginning of winter. Because solstices and equinoxes fall on different days every year, the Australian government officially recognises the 1st of December as the first day of summer, the 1st of March as the first day of autumn, the 1st of June as the first day of winter and the 1st of September as the first day of spring. I think the UK has the same system, but with the seasons reversed (it's in the Northern Hemisphere). For example, the 21st of December is the longest (in the Southern Hemisphere)/shortest (in the Northern Hemisphere) day of the year in 2015, so that's technically when summer/winter begins. The 1st of December is the official start of summer in Australia, as well as in any other countries which have this system.

What are the seasons like in Russia? This is interesting because here every season is three months.