How did Thanksgiving become a national holiday?
it became a national holiday because the felt like making it anational holiday.
When did Thanksgiving become a National Holiday and who made the Proclamation that made it a national holiday?
Abraham Lincoln signed the proclamation making Thanksgiving an annual national holiday, but Sarah Josepha Hale is actually responsible for Thanksgiving becoming a national holiday. She spent 30 years writing letters to politicians and speaking up for it to become one. Lincoln simply responded to her efforts.
Because there is another question relating to Thanksgiving which leaves out significant information, I shall answer this one. Thanksgiving was declared a national holiday by Abraham Lincoln during his first term of office. Radical Republicans were Protestants. Thanksgiving was a Protestant feast day. Lincoln appeased his Republican allies by declaring the day a US Holiday.