Some readers may wonder why a comic book that actually came out in August, 1965 has a date of October, 1965 on its cover.
Essentially, periodical publishers and distributors (then as well as now) like having their wares out on vendors’ stands as long as possible. You can still see this today with weekly magazines that come out on a Tuesday but have a cover date for the next Monday. As long as the cover date isn’t yet in the past, publishers figure that vendors may leave the books on the stands, where customers may still buy them, rather than decide that they’re old, outdated stuff and return them to their distributor for credit, even if they’ve been out a couple of months. Most modern comics still follow this convention, even though the vast majority of them are now sold on a non-returnable basis.
Also worth noting — while some comics were published monthly, 12 times a year, many weren’t. Justice League of America, for example, had nine issues published with an official publication date of 1966. (The indicia noted that the book was published “monthly, with the exception of Jan, April, July and Oct.”, but DC got an extra November-December all-reprint “80-Page Giant” in there as well.
All on-sale dates referenced on this blog are sourced from either the Grand Comics Database or from Mike’s Amazing World, which in turn draw their information from the Library of Congress’ Copyright Office records and other sources. I generally try to post about a comic book within a day or two of the 50th anniversary of its documented on-sale date; occasionally, there may be as much as a week or two’s difference, but I’ll always post about a comic in the month of its semicentennial.