This Widok miesiąca 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 4,300 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.