Is Rosh Hashanah 2021 in September or October or is it in both?

There are very good reasons for both but what tips it into the third
option for me has to do with the end of the Church Age/beginning of
the 70th Week dichotomy.

Although scholars have determined legitimacy for calendars and methods
differing from the common practice it is the common practice that has
been in force for much of the Church Age. It is only after the Church
Age ends that the the old dispensation can be restored, otherwise
there is little order and much confusion as to when, where ,what &
how.

So, if this theory be true, then it is not only Rosh Hashanah that
will be recalibrated but other things from the old dispensation as
well.

In any event,

Soon & Very Soon!
Pineman