Seasons are caused by the tilt of the earths rotational axis away or toward the sun as it travels through its year long path around the sun.

The earths position affects the temperature because it tilts. For example in June the earth will be tilted towards the sun in the northern hemisphere. Warmer weather happens when more direct rays of solar radiation shine.

The seasons are not the same in the Northern and Southern Hemisphere. This is because the days in the northern hemisphere are shorter. June 21 is the first day of winter in the Southern Hemisphere and here it's the first day of summer. December 21 is the first day of winter in the northern hemisphere and it'd the first day of summer in the Southern Hemisphere.

Spring begins between march 21 and 23. Summer starts on June 21. Fall/autumn begins on September 21 and 23. Winter starts on December 21 and 23.