Traditionally, where I'm from, it's Easter and Labor Day. The end of summer and warm weather wear is less rigidly fixed than the beginning. You wear your first pastels to church on Easter Sunday.

when the seasons change im guessing

it depends on where you live. i advocate a personal sense of style. if pastels don't look good on you, or aren't you, don't buy them, wear them just because it's spring. don't wear things just because it's the season or it's the style. I think you should be appropriate at all times, and that depends a lot on you, where you live, your age, where you're going. If you live in Los Angeles, I think it's silly to buy seasonal clothes. I think you should buy clothes that fit the generally range of temperature that work all year round. Your sweaters will look just as nice in the summer as in the winter--if both days are a chilly 60 degrees, it doesn't really matter (unless it's like a kitted cable christmas sweater with rudolph, I'd avoid that in the summer). Your shirts and skirts and pants will look good all year round. If you live somewhere where there isn't a 30 degree difference between the hottest and coldest days of the year, but an 80 degree difference, I would think you should have a seasonal wardrobe. And that it when you change it should be based on temperature. There are some people who will tell you not to wear white after labor day. If you look good in it, and it's appropriate for your job and age and weather, go ahead I'm not advocating wearing a white tanktop to work when it's snowing (or when its not, unless you work for mtv or something), or wearing white fur to work when it's warm and sunny--or when it's not. I'm against fur and it's not appropriate except for at a very few jobs. Use your head. If you want to look up the "official rules", they're fascinating, especially the history behind them, but there are many contradictory ones and they are different everyone. But still, definitely a good read. There are some very good books one the subject. Fascinating really.

When the temperature starts changing.

According to the English Seasons' starting times: 21st October 21st December 21st March 21st June!