Messages to write for Boss's Day include "I appreciate all that you do" and "You're the best type of boss." If the writer and his boss have a close relationship, he can write "Thanks for bringing out the best in me" or "I'm lucky to have you as a boss."
Messages vary depending on how close the writer is to his boss. If he doesn't know his boss well, he can write a short, formal message, such as "I appreciate the work that you do, have a great day." If he and his boss are friends, he can write a more personal message or include a joke. If his boss has served as a mentor, he can write about how much he appreciates his leadership.
National Boss's Day falls on October 16, or the nearest business day, every year. Patricia Bays Haroski created the holiday in 1958 and chose the date. She worked for State Farm Insurance Company as a secretary at the time, and her father was her boss. She chose his birthday for the date of the holiday because of how much she appreciated him as boss.
Since it's not a public holiday, not everyone celebrates Boss's Day. For those who do, a card is an appropriate acknowledgement. The boss's team could also get him a small gift or take him out to lunch.