It's an icon of a calendar, set on the 26th day of the month. It's displayed as a square with two ring-like objects at the top, to indicate that the pages flip over. There is the number 26 in the center of the square.
This icon is a part of a collection of Calendar 26 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.