The Shamsi calendar, also known as the Iranian calendar, is a solar calendar that is primarily used in Iran and Afghanistan. It is one of the oldest calendars in the world, with origins dating back to the Achaemenid Empire in the 5th century BC.

The Shamsi calendar is a highly accurate astronomical calendar that is based on the length of the tropical year, which is the time it takes for the Earth to complete one orbit around the Sun. It consists of 12 months, with each month having a variable length of 29 or 30 days. The months are named after various astronomical phenomena, such as the position of the Sun or the phases of the Moon.

The first day of the year in the Shamsi calendar is called Nowruz and falls on the vernal equinox, which is usually around March 21st. Nowruz is a significant holiday in many countries, including Iran, Afghanistan, and parts of Central Asia, and is celebrated as the beginning of a new year and the arrival of spring.

In addition to the 12 months, the Shamsi calendar also includes several important holidays and observances. These include the Day of Ashura, which commemorates the death of the Prophet Muhammad's grandson Hussein, and Eid al-Fitr, which marks the end of the month of Ramadan.

The Shamsi calendar has undergone several modifications throughout history. In the 11th century, the Seljuk Empire adopted the calendar and made some adjustments to better align it with the Islamic lunar calendar. In the 20th century, the calendar underwent further modifications to bring it into line with the Gregorian calendar, which is the internationally recognized civil calendar.

Today, the Shamsi calendar is the official calendar of Iran and is also used in Afghanistan. It is an important cultural symbol and is closely associated with Persian culture and identity. The calendar is also used by millions of Iranians and Afghans around the world to mark important events and occasions in their lives.