My friends and I just spent an extended weekend visiting the battlefield at Gettysburg. The battlefield is host to hundreds on historical markers- many union regiments placed monuments, the states have placed memorials to their fallen, the NPS placed flanked boundaries to visualize units in the field, and the War Department placed brigade/division/corps markers.
Many of these carry a plaque that chronicle the unit's actions during the 3 days of the battle. On quite of few of these, particularly the ones placed between 1880 and 1910 or so, we noticed they tended to refer to the assault from Seminary Ridge on the 3rd day as Longstreet's Assault. Popular culture calls this Pickett's Charge today.
So I suppose my question is this: When did the shift in the name occur? Did both names exist simultaneously? Was calling it Longstreet's Assault part of the Lost Cause narrative that sought to remove blame from Lee and reassign it to Longstreet?
Both titles are correct, in a manner of thinking. Lt. General James Longstreet commanded the 1st Corps, Army of Northern Virginia. His Corps, known alternatively as the 1st Corps or "Longstreet's Corps" was comprised of three divisions, commanded by Major Generals Hood, McLaws and Pickett, as well as attached artillery- see this guide for organisation
Now, the American Civil War is a bit outside my wheelhouse (I deal mostly with WWI), my understanding of the Confederate's offensive action on the third day was that Lee had detailed the centre attack to Longstreet's Corps, of which Pickett's Division was selected as the leading division for the attack.
So, while Pickett's Division was the one that went into action and gained immortality with the elan of what was to become known as "Pickett's Charge," responsibility for the attack that day was with Longstreet's Corps, as it was he who had developed the plan of attack.
It has been recorded that Longstreet was reluctant to continue the offensive, and even more so against the Union Centre positions. (See James McPherson "Battle Cry of Freedom"), wishing for the outnumbered Confederate forces to re-deploy into a defensive position in order to force the Union into a costly attritional battle.
Lee had demurred on this, as he believed that quitting the field when his army had not been decisively beaten in open battle would damage morale. (The same thing faced the Germans in 1917 when preparing to move to more defensible positions to shorten the line they held on the Western Front. more on that, and a comparison here
Ultimate responsibility for the failure properly belongs to Lee, as he famously told the men coming off the field after the charge was repulsed, "This was all my fault." Longstreet's culpability, if any is in his planning, or his lack of being able to convince Lee of an alternate strategy. However, this is all rather academic, Lee ordered Longstreet, who then ordered Pickett, and to refuse- either Longstreet refusing Lee or Pickett refusing Longstreet would have caused penalties for insubordination.
So, it is properly "Longstreet's Attack" which included, in a large part "Pickett's Charge."
Hope this helps