Company: Gekkan Sunday Gene-X

Gekkan Sunday Gene-X (Monthly Sunday Gene-X), often abbreviated as Sunday GX (サンデーGX), is a Japanese seinen manga magazine published by Shogakukan. Like many other manga magazines, it's an anthology magazine with each issue featuring new chapters of several manga series. The series are also published in book form as Sunday GX Comics.

The magazine's title, Monthly Sunday Gene-X, refers to its mission as a manga magazine for Generation X. The first issue was published on July 19, 2000 and new issues are published on the 19th day of each month — not necessarily on a Sunday. The title uses the word Sunday more as a trademark or genre name, shared with its sister magazines Weekly Shounen Sunday and Weekly Young Sunday.

The first year when the magazine was actually published on a Sunday two months in a row was in 2006. This is because 2006 is a common year starting on Sunday, the following occurrence of which is 2017. In such years, the 19th is a Sunday three times: February 19, March 19 and November 19.

The magazine is not limited to Japanese productions; a Korean comic series, Blade of the Phantom Mask, was also serialized in translated and flipped form, perhaps helped by current interest as a Japanese-Korean anime movie. If one of their manga series has such a tie-in, an anime TV series or movie version being aired or screened around the same time, editors like to emphasize this in an act of cross promotion.

Source: Wikipedia