Other than that, the date has no relevance.
was more a reply to this than a question of legality...
the whole "instrument" was pretty extraordinary. the wording of it frees those who were in servitude at the time, but did not make owning slaves illegal.
"In 1863 President Lincoln had issued the
Emancipation Proclamation declaring “all persons held as slaves within any State, or designated part of a State, the people whereof shall then be in rebellion against the United States, shall be then, thenceforward, and forever free.” Nonetheless, the Emancipation Proclamation did not end slavery in the nation. Lincoln recognized that the Emancipation Proclamation would have to be followed by a constitutional amendment in order to guarantee the abolishment of slavery."
that's what the 13th amendment did.
"The 13th amendment, which formally abolished slavery in the United States, passed the Senate on April 8, 1864, and the House on January 31, 1865. On February 1, 1865, President Abraham Lincoln approved the Joint Resolution of Congress submitting the proposed amendment to the state legislatures. The necessary number of states ratified it by December 6, 1865. The 13th amendment to the United States Constitution provides that "Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.""
so an executive order is certainly legal, but this executive order didn't make slavery illegal, it freed all those currently slaves.