'Pahela Boishakh' (or, 'Poila Boishakh') has been curved in our hearts. So I'm taking quite a risk to write this note!I believe, the words 'pahela', 'dosra', 'thesra', etc. have been borrowed from Pharsi/Urdu. The correct Bangla words for these are 'prathama', 'dwitiya', 'tritiya', etc. The names of the months of Bangla calendar 'Bangabda' are based on the names of the নক্ষত্র (lunar mansions) - locations of the moon with respect to particular stars during the lunar cycle. These names were derived from 'Shakabda', another calendar of this region that was introduced during the Shaka Dynasty. The dates of these months were referred to as 'prathama', 'dwitiya', 'tritiya', 'chaturthi', 'panchami', 'shashthhi', 'saptami', 'ashtami', 'nabami', dashami', etc.
I believe, when the Emperor Akbar revised the Bengali calendar to synchronize it with the Hijri, the Pharsi/Urdu terms 'pahela', 'dosra' etc. came along.
It is very surprising that, both 'Ekushey February' and 'Pahela Boishakh', the two very important events of the Bangalis, are referred to by non-Bangla words! While Ekushey should be referred to as 'Aat-i Phalgun' , the first day of Boishakh should be 'Prathama Boishakh'. Though the adaptation of the foreign word 'Pahela' as 'Poila' has minimized the linguistic sin, it wasn't really necessary as Bangla already had a perfectly good corresponding word. It is not like 'Akademi' which is used in West Bengal as Calcutta University adapted the English word into the language, much like 'haspatal' for 'hospital', because there were no suitable corresponding words.
Anyways, I hope I have not poured cold water onto the high spirit of the Bangla New Year. Shubha Nababarsha to you all, and have a Shubha Prathama Boishakh!!