They're posted in the Boston Aviation thread on airliners.net every month.
Typically, on the second or third Thursday of the month - Bureau of Transportation Statistics posts the monthly data from six months ago.
https://www.transtats.bts.gov/DL_SelectFields.asp?Table_ID=261&DB_Short_Name=Air Carriers
You just have the select the information (seats/passengers) and the time period you want.
Next week's release will be November 2016