April 5, 1941. Herringbone tweed and Camp Lejeune.

On this day in 1941 the U.S. Army adopted the herringbone tweed cotton fatigue uniform, which I know only because I read that here:

Today in World War II History—April 5, 1941

This history of the uniform, which was adopted in order to replace the Army's blue denim fatigue uniform, can be found here;  

The WWII Army HBT Uniforms

It was widely used by the Army in combat in warmer climates, such as in the Pacific, the China Burma Indian theater, and Italy.

Brigadier General Frank Merrill of Merrill's Marauders with two of his Japanese American interpreters.  The interpreters are wearing HBT uniforms.

The Marine Corps would follow and also adopt a HBT pattern uniform, which is the one that Marines are nearly universally depicted wearing in combat during the war.

Navy corpsman and Marine on Guam.

A camouflage variant would go on to be developed for the Army, but because it tended to confuse American troops who associated camouflage with the SS, it was withdrawn from service and issued instead to the Marine Corps, which then went on to adopt its own widely used variant.

On this day also, Congress authorized the construction of the Marine Corps facility, Camp Lejeune.

Marine Corps corporal training at Camp Lejeune.  He's firing the M1941 Johnson Machine Gun which saw limited use with the Marine Corps until adequate numbers of the Browning Automatic Rifle could be produced.