” In 1863 Thanksgiving Day was proclaimed a legal holiday. “
1941
According to Wikipedia, thanksgiving became an official Federal holiday in 1863, when, during the Civil War, President Abraham Lincoln proclaimed a national day of “Thanksgiving and Praise to our beneficent Father who dwelleth in the Heavens”, to be celebrated on the last Thursday in November.
IIRC, in 1941 the observance was standardized to the fourth Thursday, rather than the last Thursday as Lincoln had proclaimed it. A running joke had it that diehard Republicans refused to observe the new date & politically divided families thus celebrated two Thanksgivings.
Thanksgiving was made a legal holiday in 1178, under the Monarch Kilij Arslan II, in the Sultanate of Rûm.