there was only one calendar 2000 years back

Gregorian	Hindi original months	old name	tamil name
january	Gyarah amber	Pausa	thai
february	Barah amber	Magha	masi
march	Ek amber	Phalguna	panguni
april	Tho amber	Chaitra	chiththirai
may	Theen amber	Vaisakha	vaikasi
june	Char amber	Jyaistha	ani
july	Panch amber	Asadha	adi

august	Cheh amber	Sravana	avani
September	Sapta amber	Bhadrapada	purattasi
October	Ot amber	Asvina	aippasi
November	Nov amber	Karttika	karthigai
December	Das amber	Margasirsa	margali

After julius ceaser and augustes no body changed the names and the names saptamber and other three months are escaped. Gregorian shifted the first month to january, other wise april will be the first month and branded the people who are celebrating the april as first month as fools and we are celebrating that also now.
Now tamilnadu government is celebrating thai as the first month which is january.