The word Agrahāyaṇa means the month of Ayana or Equinox (agra=first + ayana = travel of the sun, equinox). The aligning of this name with the MṛgaśirasNakṣatra(lambda orionis), gives rise to speculation that this name may have been given when the sun was near Orion at the time of vernal equinox, i.e. around 7000 years ago.
In lunar religious calendars, Agrahāyaṇa may begin on either the new moon or the full moon around the same time of year, and is usually the 9th month of the year.
In solar religious calendars, Agrahāyaṇa/Maarkazhi begins with the Sun's entry into Sagittarius, and is usually the 9th month of the year.
Kālabhairava Aṣṭamī (or Kālabhairava Jayanti) falls on Kṛṣṇa PakṣaAṣṭamī of this month of Mārgaśīṣa. On this day it is said that Lord Śiva appeared on earth in the fierce manifestation (avatāra) as Śrī Kālabhairava. This day is commemorated with special prayers and rituals