I graduated high school in 1980, and as a second career, am helping HS students with their math. I just finished my 6th year of this, and have just one question about the change noted in the title.
I recall Septagon, Octagon, Nonagon, Decagon, being figures with sides numbering 7,8,9,10. As a child, I noted that these prefixes reflected those of the last 4 months on the calendar. From this a teacher explained that the calendar months got shifted around, and that at one time September thru December were, in, fact, months 7-10.
A few years ago, I ran into a seven sided figure and a student telling me it was a Heptagon. I do understand that Sept is Latin and Hept is Greek, but would like to know when and more important, why, this change was made.