What astronomical event corresponds to the passage of one month?

The moon revolves around the earth once, and we observe the cycle from new moon to full moon.

That lunar cycle takes approximately 29 1/2 days.  So if we were still to follow a lunar calendar, we'd lose about 11 days per year compared to the Gregorian calendar which we follow.  The Gregorian calendar is named after Pope Gregory XIII, who introduced the calendar in 1582 as a reform to the Julian calendar (named after Julius Caesar), which had been followed since Roman times.  

A historical item of interest about the Gregorian calendar:  Because Russia was dominated by the Orthodox Church and opposed to anything a pope had introduced, Russia continued to use the Julian calendar until 1918, after the communist revolution changed the governing authority in Russia.

The  pan-african movement  was an "emotional, cultural, psychological and ideological  movement  that began among the  african  dispora in the western hemisphere, for a closer  purpose, so that  african  people could feel secure, attain political, economic as well as psychological power visa vis other races or world regions" .

Iwould like a photo instead of all this and you can’t just go out asking for a paragraph thinking someone will be like oh yeah here it is