The astronomical and calendar year
The Earth travels around the Sun in an orbit, and its axis is slightly tilted. So during the year sunlight reaches the Northern Hemisphere at different angles. In summer the light falls more directly: it is warm and days are long, nights short. In winter the light falls at a slant: it is cold and days are short. Spring and autumn are in between. Around 21 March and 23 September day and night are equal (equinox). The Earth circles the Sun in 365 days and about 6 hours. So every four years one day is added to February: that is a leap year with 366 days and 29 days in February. A leap year is usually divisible by 4 (2024, 2028). A common year has 12 months and 365 days.
Experiment with a torch and a globe: darken the room and use the torch as the “Sun”. Hold the globe slightly tilted, move it around the torch and observe when the Northern Hemisphere is lit most brightly. Then make a year calendar with your family’s birthdays.