In ancient Roman religion and myth, Janus (/ˈdʒeɪnəs/; Latin: Ianus, pronounced [ˈjaː.nus]) is the god of beginnings and transitions, and thereby of gates, doors, doorways, passages and endings. 

He is usually depicted as having two faces, since he looks to the future and into the past (history) of every person’s life according to the Roman calendar (Gregorian calendar).

The reason why Janus is depicted with two faces:
The rituals of serving Janus included the using of divination and horoscope to look into the people’s list of history (containing the people’s fears, weaknesses and shortcomings), and using the method of invoking and introducing new policies, products via commercial, advertisements, propaganda in media, which was aimed at putting the people into the act spending money on material things as a solution which will help them overcome their fears and secure their future.

It is conventionally thought that the month of January is named for the Roman pagan god (deity) called Janus.
As a god of transitions, he had functions pertaining to birth and to journeys January -representing the Roman god Janus and exchange, and in his association with Portunus, a similar harbor and gateway god, he was concerned with travelling, trading and shipping.

January introducing new habits of money spending into people’s lives:
While December represents death, dark troubles of winter, warfare or the expiry date of things which confronts humanity, but on the other hand; January represents the beginning or the putting in motion of new things. 

As a god of motion, the pagan god Janus looks after passages, causes actions to start and presides over all beginnings.
This is the reason why most of the beginnings which introduces new products, new changes in politics, the returning of the people from their Christmas vacation back to work, occurs during the month of January.

In fact; in this month there is a lot of harassments and enforcing of new policies, products from vendors, different ways how money should be spent, on what it should be spent on etc, all these new changes are enforced upon the people as new habits, using January as the beginning and introduction phase which introduces the people to these new life changes.