According to the National Education Agency website, there was a National Teacher Day in 1953 and again in 1980, as proclaimed by the U.S. Congress. In 1985, the NEA voted to change the observation and celebration to the Tuesday of the first full week in May.
That's where this year's confusion comes from. The first day of May was on a Monday this year. That's the first day of the school week, but Sunday is the first day of the traditional calendar in the United States. So, some people and schools are celebrating National Teacher Day and Teacher Appreciation Week this week.
However, the National Education Agency and the National PTA have announced that the events are next week, May 7-12. National Teacher Day will be May 9, 2017. And, that's good for letter writers who haven't yet written a letter to their favorite teacher! There's still time! You can write a letter to your favorite teacher from when you were in school, or if you have kids of your own now, you can write a letter of appreciation to one or all of their teachers.
If you need something to get your letter started, the NEA and PTA presidents have posted their own letter of appreciation online, and the PTA has a downloadable thank you card that parents can printout for their kids to give to teachers.