Our concept on days, months, and years are all based on the orbital cycles of the moon, Earth, and Sun. Calendars are used to take mark on what day, month, or year it is.
A day is the time it takes the Earth to spin around once. It is day time on the side of the Earth that is facing the Sun. When it is night time, that side of the Earth is facing away from the Sun. It takes 24 hours for the Earth to spin once, so that is one day, including the day time and night time. This is mean solar time, measured relative to the Sun. There is also sidereal time, measured in respect to the fixed stars. The sidereal day is four minutes shorter than a solar day.
A month is about 1/12 of a year, and usually has 30 - 31 days (except February which has only 28 - 29 days). It was originally based on a period of time between two full moons.
One year is about 365 days long, except in a leap year. It is the time it takes the Earth to orbit the Sun once. A year is actually 365,24 days long, but a calendar has 365 days, which is why there are leap years every four years. The ,24 is multiplied by four to give an extra day every leap year (February 29). However, due to the extra day period being only 0,24 , leap years days are skipped once in every 100 years.
Seasons relate to specific points in Earth's trip around the sun. When it is summer in the northern hemisphere, the Earth's axis is tilted so that the northern hemisphere is tilted towards the Sun, giving the northern hemisphere more heat and light and making it winter in the southern hemisphere. The vice versa occurs when it is winter in the northern hemisphere, making it summer in the southern. Spring and fall occur when the Earth's axis is tilted so that the equator is pointing towards the Sun. The solstices define the start and end of a season, the summer solstice is June 21 and the winter solstice is December 21.