Is there really a 13th month? I have not found and biblical evidence of this being true. I know that there are those who want to first have the barley be aviv before beginning the first month. However, why couldn’t the barley become aviv in a very short period of time. YHVH is certainly capable and able to do anything. Having barley become aviv in a matter of a few days, hours, minutes or seconds is not something that’s above his ability.
Then this would mean that we have celebrated the spring and fall feasts a month behind all year long. Year by year, and possible for several decades or centuries.
I just found in researching a different issue all together, that the Sanhedrin added the 13th month to the year from time to time to assure that Nissan does not fall in the winter. this is found in my TANAK in the commentary on page 466, a comment by Rashi.
So the 13th month is NOT biblical and is a practice that I do not accept.