This is a Widok miesiąca 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 12,100 icons, all in the same style and quality.
This is a series of rectangles in which you can draw in whatever you want into the days. It's a bunch of lines going criss-cross, as in one goes perpendicular and the other vertical. They do this until they make a pattern in which can be written in between.