From our inquiry into the derivation of the Tamil margazhi from Margasirsi, you must have formed an idea of how the genius of one language differs from that of another. You may note this from how the original Sanskrit names of other months have changed in Tamil. Usually, as observed before, the name of a month is derived from the asterism under which the full moon falls in that month. Citra-purnima is a sacred day. The Tamil Cittirai does not represent much of a change from the Sanskrit "Citra".
Vaishaka is connected with the asterism Visakha; it is "Vaikasi" in Tamil. Just as Madurai becomes Marudai, so the Sanskrit, Vaishaki has changed to "Vaikasi" in Tamil. (In Bengal the month is called "Baisakhi", )Visakha is the asterism under which Nammazhvar was born. Now Vaisakha purnima is celebrated as Buddha purnima.
The month Anusi is associated with the asterism of "Anusa"[ Anuradha]. The full moon usually falls under this asterism during this month. In Tamil the month is called "Ani"- the "sa-kara" of the original has dropped.
There are two "Asadhas"- Purvasadha and Uttarasadha (Earlier Asadha and later Asadha). Purvasadha is called "Puradam" in Tamil; in the Tamil name the "rva" of the original is eroded and the "sa" has dropped. Similarly, Uttarasadha is "Utradam "in Tamil. The Sanskrit "Asadhi" is the Tamil month of "Adi".
Sravana means that which is associated with the asterism Sravana. In the Tamil "Onam" the "sra" of the original has dropped and "vana" has become "onam". Since it is the asterism sacred to Mahavisnu the honorific "Tiru" [equivalent of Sri] is prefixed to its name --thus we have "Tiruvonam". (Ardra is the asterism sacred to Siva. It is called " Adirai" in Tamil and with the prefixing of "Tiru" it becomes "Tiruvadirai". It is not customary to add " Tiru" to the Tamil names of other asterisms. In the South, the is a festival of lights in the month of "Karttigai" --the original Sanskrit name is Krttika. During this time alone is " Tiru" added to "Karttigai". But to the asterisms sacred to Hari and Hara-- Visnu and Siva--"Tiru" is added. Here is proof of the fact that it is part of the religious culture of Tamils not to maintain any distinction between these two gods). To come back to Sravana. The full moon in this month generally falls under the asterism of Sravana. In the Tamil name of "Avani", the " sra" of the original has dropped.
For this linguistic phenomenon of letters dropping off in Tamil there is the example of "Izham" for Simhala [the island nation known as Sri Langa]. "Sa" and "sa" become "a" in Tamil. If "sahasra" is "sasiram" in Kannada, it is "ayiram" in Tamil.
"Ayiram" reminds me of other numbers. The Tamil numbers onru, irandu, mundru (one, two, three)seem to have no connection with the Sanskrit eka, dvi, tri. But ancu and ettu (five and eight) seem to be related to the Sanskrit panca and asta. The English "two" and "three" are related to the Sanskrit dvi and tri. Sexta, hepta, octo, nano, deca -- these are obviously connected with the Sanskrit sasta, sapta, asta, nava and dasa. But the very first number "one" seems totally unrelated to the Sanskrit "eka". But, strangely enough, it appears to have some connection with the Tamil "onru". The Telugu equivalent is made up of the "o" of the Tamil "onru" and the "ka" of the Sanskrit "eka" -- "okati". If we consider all this, just as we are one racially, in the matter of language for Sanskrit and Dravidian tongues.
In Simhala the "sa" and "ha" of "Simha" have dropped off and the word has become "Ilam" and the "la" has changed to "zha" to become "Izham".
Like Asadha, Prosthapada has also a Purva and an Uttara. Purva-Prosthapada is " Purattadi" in Tamil: "asta" changing to "atta" is already known to us. Uttara- Prosthapada is "Utrattadi" in Tamil. The full moon falls under this asterism or the one near it in the Tamil month Purattasi which name is derived somehow from Prosthapadi.
We call Asvayuja Asvini or "Asvati". The full moon conjoined with the asterism Asvayuja makes the month Asvayuji which in Tamil is "Aippasi".
The "Karttika" of Sanskrit (adjective of Krttika)has not changed much in its Tamil equivalent of Karttigai. The "Tirukkarttigai" festival of lights usually falls on a full moon. I stated with how Margasirsi changes to "Margazhi". The full moon of that month is celebrated as Tiruvadirai, the day sacred to Siva.
"Pusya" is the Tamil "Pusam". (We in Tamil Nadu have got so used to "Pusam" that we have made the asterism "Punarvasu" into "Punarpusam". Of course there is no Sanskrit equivalent like "Punarpusya") "Pausya" means what is associated with Pusya. Pusya is also known as Taisya. The Tamil name of the month "Tai" is the result of the second syllable of "Taisya" dropping off.
The month "Magha" is named after the asterism Magha -- in Tamil it is "Masi". The "si" ending is reminiscent of "Vaikasi", "Purattasi" and "Aippasi".
There are two asterisms called Purva-Phalguna and Uttara-Phalguna. In the corresponding Tamil names the important part of the Sanskrit original, "Phalguna", has dropped off. So "Purva-Phalguna" is mere "Puram" in Tamil and "Uttara-Phalguna" is mere "Utram". But the month in which the full moon falls under the asterism of Uttara-Phalguna is "Panguni" for Tamils. It is a festive day in many parts of the south. We celebrate it as Panguni-Utram Tiruk-kalyanam.
From an examination of the Tamil names of the months we form an idea of how the phonemes of Sanskrit change in Tamil.
For a general background, please see here