Because Presidents Day is not the official name of the federal holiday, there is variation in how it is rendered. Both Presidents Day and Presidents' Day are today common, and both are considered correct by dictionaries and usage manuals. Presidents' Day was once the predominant style, and it is still favored by the majority of significant authorities—notably, The Chicago Manual of Style (followed by most book publishers and some magazines), The American Heritage Dictionary of the English Language, Webster's Third International Dictionary, and Garner's Modern American Usage. In recent years, as the use of attributive nouns (nouns acting as adjectives) has become more widespread, the popularity of Presidents Day has increased. This style is favored by the Associated Press Stylebook (followed by most newspapers and some magazines) and the Writer's Digest Grammar Desk Reference (ISBN 978-1582973357).
President's Day is a misspelling when used with the intention of celebrating more than one individual (see also apostrophe); however, as an alternate rendering of “Washington's birthday,” or as denominating the commemoration of the presidency as a singular institution, it is a proper spelling. Indeed, this spelling was considered for use as the official federal designation by Robert McClory, a congressman from Illinois who was tasked with getting the 1968 federal holiday reorganization bill through the House Judiciary Committee. Nonetheless, while Washington's Birthday was originally established to honor George Washington, the term Presidents Day was informally coined in a deliberate attempt to use the holiday to honor multiple presidents, and is virtually always used that way today. Though President's Day is sometimes seen in print — even sometimes on government Web sites, this style is not endorsed by any major dictionary or usage authority.