Final Answer:
The first Attack on Titan magazine was published on September 9, 2009.
Step-by-step explanation:
The Attack on Titan manga series, written and illustrated by Hajime Isayama, first appeared in Kodansha's Bessatsu Shōnen Magazine. The initial chapter was published in the September 2009 issue, officially released on September 9, 2009. To determine this, one can refer to historical records, official announcements, or the publisher's statements regarding the release date of the manga.
The significance of this date marks the beginning of the serialization of the popular Attack on Titan series, which gained widespread acclaim and contributed significantly to the manga and anime industry.