What President made Thanksgiving a legal holiday?

The Continental Congress issued the First National Proclamation of Thanksgiving in 1777. George Washington declared Thanksgiving Day as a national holiday in 1789 and again in 1795.
John Adams declared Thanksgiving Days in 1789 and 1799.
No Thanksgiving proclamations were issued by Thomas Jefferson but James Monroe declared a Thanksgiving Day in 1814 and twice in 1815.
There were no more national Thanksgivings until 1863 when Lincoln designated one on the final Thursday of November. All the following Presidents annually declared this date as Thanksgiving day until 1939 when there were five Thursdays in the month and Franklin Roosevelt decided to declare it on the fourth Thursday. In 1940 and 1941 he proclaimed the third Thursday as Thanksgiving. Since a Presidential Proclamation is not legally binding, 23 states celebrated on the third Thursday, 22 celebrated on the fourth, and Texas took both days off!

In 1941, Congress passed a joint resolution fixing the traditional last-Thursday date for the holiday beginning in 1942. A Senate amendment to that resolution changed the date to the fourth Thursday. President Roosevelt signed this bill on December 26, 1941, making the date of Thanksgiving a matter of federal law.