I remember the DC Giant Schedule being one of the great constants of my childhood (along with the fact that only Adventure, Action and Detective were monthly, most of the other DCs were 8 times a year and I could probably still tell you the skipped months of each title). I guess that part of my growing up involved the crushing revelation that these schedules and publication frequencies were not set in stone and would change without warning, even that series that I followed could be cancelled entirely!

Anyway, as I recall, by 1966 the DC Giant Schedule was

January: Batman
February: Our Army at War (featuring Sgt. Rock and the other war series)
March: Action Comics (featuring Supergirl reprints)
April: The Flash
May: Superboy
June: Superman
July: Batman
August: Jimmy Olsen
September: Lois Lane
October: World's Finest
November: Justice League of America
December: Superman