"Jupiter, the biggest planet in our solar system is called as 'Guru Graham' in Sanskrit. This planet is related to the intellect of humans. Hence it is called as Guru. The planet Jupiter, changes from one Raashi (12 astrological signs. Aries, Taurus, etc) to another every year. During the period of retrograde it slows down but still it makes up for the slowness and on an average it changes the Raashi house every year. This change generally happens in the last week of the month December. This type of calendar system is connected with the movement of the Sun and is called as Solar Calendar."