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Octa means 8The word Octa is derived from Sanskrit word 'Ashta' which means 8. :)
Let me try to comment on october, which is supposed to be 10th month of the year.

Septa, meaning 7 ( derived from sanskrit Sapta), So September means 7th month
Octa, meaning 8 ( derived from sanskrit Ashta),So October means 8th month
Novem, meaning 9 ( derived from sanskrit Nava)So November means 9th month
Deca, meaning 10 ( derived from sanskrit Dasha)So December means 10th month

Having this logic continued Jan and Feb will be 11th and 12th month respectively. This means March will be First month. But somehow, in the history, this convention of starting the year from March has been changed to Jan. Actually Indian Calendar starts from around Mid March. Also even today Financial Year in India starts from March.