We know that the moon makes 12/13 revolutions around the earth
producing 12/13 full moons
from a particular time like the summer solstice to the next.
The question that comes to my mind is
what did the Arabs call the 13th full moon
in Prophet Muhammad's time?
http://astropixels.com/ephemeris/phasescat/phases0601.html01 -> Jun 02, 619: Ramadan
02 -> Aug 01, 619: ...
03 -> Aug 30, 619: ...
04 -> Sep 29, 619: ...
05 -> Oct 28, 619: ...
06 -> Nov 27, 619: ...
07 -> Dec 26, 619: ...
08 -> Jan. 25, 620: ...
09 -> Feb 23, 620: ...
10 -> Mar 24, 620: ...
11 -> Apr. 22, 620: ...
12 -> May 22, 620: ...
13 -> Jun. 20, 620: ?
01 -> Jul 20, 620: Ramadan
.......
The question is related to what extent the 12-moon lunar year can be taken seriously,
according to which the 4 restricted full moons are not consecutive.