This Calendar 21 icon is made in Material Design style developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 4,600 icons, all in the same style and quality.
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.