From the most ancient times, all religious activity of the Hindus was performed at times considered auspicious or suitable for them. Calculations were done based on the movements of the sun, the moon, the planets and the stars. This resulted in the growth of an entire science of Vedic Astronomy, ‘Vedāṅga Jyotiṣa’.
The most fundamental periods of time, common to all the nations are the day, the month and the year. The day is due to the revolution of the earth round its axis. The month is mainly a lunar phenomenon based on the waxing and waning of the moon. The year is due to the apparent motion of the sun as seen from the earth. Hence the Hindus followed a luni-solar calendar from the ancient times, and being aware of the difference between the two ways of calculating, also provided for the necessary adjustments to be made to bring the two into consonance. The lunar year on which are based the religious activities, is less than the solar year by about eleven days. Hence two intercalary months are added in five years, one being a second āṣāḍha (the 4th lunar month) at the end of 2 years and a second pauṣa (the 10th lunar month) at the end of another 2 years. This is according to Vedāṅga Jyotiṣa. Later works give varying periods after which the intercalary month occurs. The general rule seems to be that the lunar month in which no saṅkrānti (apparent movement of the sun from one constellation to the next, as seen from the earth) occurs, is called an adhikamāsa and bears the same name of the next lunar month which is called śuddha or nija or prākṛta, to distinguish it from the former.
Somehow, the adhikamāsa was considered even from the ancient times, inauspicious and hence named malamāsa (‘the dirty month’), malimluca (‘a thief’), aṁhaspati (‘the lord of sins’) and so on. During this month one was advised to avoid acts like consecrating Vedic fires, installing images, offering gifts, observing vratas or religious vows, performing upanayana and other saṁskāras (puri-ficatory rites).
However in some purāṇas like the Padma (6.64) the intercalary month is named ‘puruṣottama-māsa’ (‘month of Lord Viṣṇu’), probably with a view to reducing the opprobrium attached to it and considered, on the other hand, as holy.