This is a Calendar 26 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 12,800 icons, all in the same style and quality.
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.