This document provides instruction on using the prepositions "at", "on", and "in" to indicate specific times and periods of time in English. It explains that "at" is used to refer to specific times on the clock or expressions like "at night". "On" is used for days of the week, specific dates, or periods like "on the weekend". "In" refers to longer periods like months, seasons, or parts of the day like "in the morning". Examples are given for the different uses of each preposition.