2021 calendar. 2021 will not be a leap year, it will have 365 days. It will start and end on a Friday. It will be the 2nd year of the twenties (‘twenties’ for short). It will be in the 21st century. The first calendar week of the year will start on Monday 4th January. This year there will be 52 calendar weeks.
One year is the time it takes the earth to make one orbit around the sun. The month was originally based on the orbit of the moon around the earth, which takes 29.5 days (see lunar calendar ). The four weeks of a month probably originally referred to the four phases of the moon. According to ISO 8601, the first calendar week (CW) of a year is the first that has at least four days. A day is the distance between two comparable positions of the sun; it begins and ends at midnight. For the names of months and days, see Calendar Names .