Sun’s annual entry into sagittarius is denoted as Margashirsha month. Every year from mid december to mid january sun will be in sagittarius based on the sidereal solar calendar. Sagittarius consists of 3 nakshatras Mula, Purva Ashadha and Uttara Ashadha. The 7th aspect of Mula falls directly on Mrigashirsha nakshatra (every nakshatra has default 7th house aspect like every planet does), thus the name Margashirsha. Theme of this month is related to Mrigashirsha nakshatra such as searching for higher knowledge, new beginnings, becoming more active, taking steps and actions, communication, searching for deep hidden truths and meanings etc.
Sun and Moon are the significators of king and queen, father and mother, ether and earth, left and right brain, abstract and logic respectively. The energy of all the different facets of astrology can be felt through the Sun and Moon itself. There can be no earth without sun and moon. That's why they both are given this utmost importance in jyotish. Their relative positions give a lot of indications to understand the current events. That’s why no matter how many different calendars we have around the world everything is based on lunar, solar or luni-solar calculations.
Religious and spiritual significance
Margashirsha has a special religious significance among months. Our one year is one day for the devas based on their dimension (according to vedas). Their dawn occurs everyday at the time of our Margashirsha month every year. Dawn is also called Brahma Muhurta, the most auspicious time of every day when the sattvic energy is at its maximum.
Sun moves toward the southern hemisphere from july to december as seen from the tropical region and moves toward the north from jan to june every year based on the sidereal timeline. Margashirsha marks the midway when the sun appears to be stationary and moving towards the north (the daylight in the tropics will be more from jan onwards till june). It exactly starts from the day when moon transits Ardra (Betelgeuse) nakshatra in this month.
Read more about the Ardra nakshatra and Arudra darshan ever year.
Shastras (dharma, artha and kama) talk about the three gunas that are the basis of everything in the world. Sattva, Rajas and Tamas. Sattva is responsible for good things, rajas is for the actions and efforts based on the ego, tamas is for the illusion, anger and greed. Everyday is filled with the mix of these three gunas. Guna means habit or the underlying factor of one’s actions. Sattva time is from dawn to early morning, rajas is from morning to evening and tamas is from dusk to the time before dawn (roughly till 3am). That's why 3am gets the front seat in every horror journal.
Lord Krishna in Mahabharata said that he’s the spring among seasons, Margashirsha among months and Mango among fruits. The reason being the benefits it has for the good of mankind. Since Margashirsha represents the Brahma Muhurta time of a year it is recommended to start all the sattvic things such as pujas, homas, yogic practices, charity works, learning etc. especially from the Ardra nakshatra day.