There is a story about the Asura, Tripurasura. This Asura was demonic in nature, filled with aggression. He was motivated toward yogic practice for purely selfish reasons. He held the asana of standing on only one leg for a hundred years, in deep meditation on Brahma. The he stood on his three heads for a thousand years, in deep meditation on Brahma. This demonstration did not go unnoticed, and attracted the praise of Brahma. Brahma asked, why do you meditate on Me? Tripurasura said, I want you to make me immortal. Brahma himself was not immortal, so he said this was not something he could do. Would Tripurasura ask for something else? Tripurasura thought, and asked Brahma to build him three fortresses: one of gold in space, one of silver in the sky, and one of iron on earth - and not permit Tripurasura to die until someone could destroy all three fortresses with a single arrow.
Each of the forts were as big a city, and the Asuras flourished in them under Tripurasura. The Devas, who are perpetually opposed to the Asuras, did not like this at all: their rivals were now stronger than they ever might be! The Devas sought the help of Shiva in destroying the Asuras - but he would not help, as the Asuras had done nothing wrong, except flourish. Why were the Devas bothered by this, anyway? Shiva told the Devas they should celebrate with the Asuras and become friends. Vishnu told the Devas the same thing. Until the Asuras did something wrong, Vishnu would do nothing. Why were the Devas afraid, anyway? The Devas said they were terrified that, despite the recent peace between the Devas and the Asuras, the Asuras would do something wrong, and hurt them (such things had happened in the past).
Vishnu saw some reason in this fear, for in the past the aggression of the Asuras had caused many problems for the Devas. So he sought to see if the Asuras were still capable of aggression. Vishnu created a human to see if the Asuras might be confused: this human would teach there was no difference between heaven and hell, and that both existed between space and earth - really, had the Asuras ever seen anything else? The human would teach that there was no consequence for our good or bad actions, before or after death, no difference between good and bad at all. The human would teach so many things contrary to the Vedas, even intoxication, even that the Asuras were superior to all other beings, and thus convince the Asuras to do wrong against the Devas. Sure enough, the Asuras were easily confused - and harassed the Devas, providing Shiva and Vishnu the excuse to produce the arrow and bow required to destroy the three fortresses. Ever since then, the Asuras have distrusted humans.
There are many lessons meditated upon during Karthika Masam. All beings are the same in that they are equal, even if they are different in nature - neither superior nor inferior,but different.
There are numerous rituals to help in this meditation on sameness and difference.
A bath is typically taken, scented candles lit in the morning and evening at entrance to the house (transforming the entire home into a shrine), there is vegetarianism. Stargazing and moonwatching is undertaken. Sadhana is sought. The herb, basil, is consumed and given in offering, typically by burning (basil was a woman who loved Vishnu very much, and wanted to become his wife - but Lakshmi would not bear her competition, and so turned her into a plant. However, the plant remains dear to Vishnu). Bathing by moonlight or at night helps the contemplative moment: just as a horse is best trained after being broken (exhausted) by exercise, at night the mind is tired enough to be more easily trained. The stars and moon help us understand the limits of our understanding; a study of astronomy helps us understand the orderly nature of our own existence, and causes faith that enlightenment is possible - and inevitable to those who strive for it.
The reasons behind these rituals are simple. The bath helps in contemplation, and relaxation, purifying body of toxins (internally and externally) and the mind of its own poisons. It is also the time in India when the new rains flood the rivers, and it is an opportunity to bathe with water freshest from its source - the sky. The flooding purifies the river. This is a way of affirming that we want to get as close to the pure truth as possible.
The candles help keep away insects, and protect the home from disease and discomfort - as knowledge and wisdom does. Lighting them at dawn and dusk is timed with the appearance of these insects. The basil, too, has medicinal properties, being antimicrobial.
Vegetarianism also helps purify body and mind, boosting the immune system. Some people fast, as well, for the same reason. The effort is made to make the body strong enough for accomplishing the purpose of Yoga.