This is a Calendar 11 Filled 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 13,000 icons, all in the same style and quality.
It is the number 11 contained within a rectangle. It appears to be a page from a calendar that is bound at the top. There are two binding rings represented by two small rectangles on the top. The date, which is 11, is written on the page below the binding.