This is a Thursday 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.
Thursday is represented by the first two letters of the word Thursday, Th, which are centered in a square that resembles a page from a day calendar, with two squares on top to look like hangars for the calendar.