This is a Calendar 21 icon. We've made it in iOS style, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 12,300 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.