This is an icon for a square shaped calendar. At the top of the calendar are two rings where each page of the calendar would flip over to the back of the pages. On the calendar is a large '21' for the day meaning that this is not a monthly calendar, but a daily calendar.
This icon is a part of a collection of Calendar 21 flat icons produced by Icons8. Icons follow the guidelines of iOS, Windows, and Android and are designed by a single designer, guaranteeing the consistent quality.