Most of your peer review journals as well as popular science/history periodicals are published FOUR TIMES PER YEAR at roughly equal intervals.
There's a USPS rule regarding the minimum number of times you need to publish to be recognized as a Periodicals Class publication entitled to the lower Periodicals Class rates of postage.
That minimum number of times is FOUR. It's been in place for over a century!
So, publish in December ~ then publish in April ~ then publish in July ~ then publish in September ~ you are hitting some peak times in America's public life ~ to wit: Christmas, Easter, Mid-Summer and Fall Semester/Harvest. This is a very popular schedule particularly since you can expect your readers to have some Holiday Free Time to read your publication. With that seasonality in place it's a logical step to do the Bible/Rome/Greece stuff in December. Other related materials gravitate around the other holiday periods.
I hate when my conspiracy theories are punctured with facts!!!!!!!