Presidents' Day, also known as Washington's Birthday is a federal holiday in the US that celebrates the life of the first President of the US, George Washington.
Although the day was originally in honour of George Washington, today, it's more widely recognised as a day to honour all past US Presidents, which is why it's commonly referred to as Presidents' Day.
Presidents' Day is a federal holiday in most states, but not all of them. It's also up to the individual state whether they officially call the holiday Presidents' Day or Washington's Birthday, and whether they celebrate in honour of George Washington or Washington and other presidents.