Prepositions of time connect an event to a moment, a day, a period, or a limit in time. They show when something happens, how long it lasts, or how far a time span extends. Time expressions without a preposition also act as time markers when the word itself already signals the moment.

Use in for months, years, centuries, longer periods, and many parts of the day. It is the normal choice for general time periods, but at night is fixed while in the morning and in the afternoon are normal. In British English, at the weekend is common, while American English usually prefers Common Prepositions.

Idea	Example
Use in for months, years, centuries, and long periods.	She arrived in July.
Use in for parts of the day such as morning, afternoon, and evening.	We study in the morning.
Use at night for the night period instead of in night.	They work at night.

Use on for days, dates, and named calendar days. It is the normal preposition for a specific day on the calendar, including birthday references when the birthday is treated as a date. For holidays, usage can depend on specificity, so at Christmas and on Christmas Day both appear in English.

Idea	Example
Use on for days of the week and calendar dates.	The meeting is on Monday.
Use on for specific date expressions.	We met on July 4th.
Use on for a birthday when the day is treated as a date.	I visit her on my birthday.

Use at for exact clock times, precise points in time, and some holidays. It is the most specific of the common time prepositions and often marks a single moment rather than a period. At noon and at midnight are fixed expressions, and at Christmas is common when the holiday is viewed as a season or occasion.

Idea	Example
Use at for clock times and exact moments.	The train leaves at 3:00.
Use at for noon and midnight.	Lunch is at noon.
Use at for some holidays and celebrations.	We gather at Christmas.

Use by for a deadline that means no later than a time, and use until or till for a time that continues up to a limit. By points to the latest acceptable moment, while until and till describe duration that stops when the limit arrives. From and to or until can frame an explicit range when both endpoints matter.

Idea	Example
Use by for a deadline or latest time.	Submit the form by Friday.
Use until or till for an action that continues up to a point.	Wait until midnight.
Use from and to for a full time span with two endpoints.	The office is open from 9am to 5pm.

Use for to show how long an action or situation lasts, and use since to show the starting point of something that continues to the present. For describes length, while since introduces the time when the action began. During names a period in which something happens sometime within that span, not the full length of it.

Idea	Example
Use for to express duration length.	She stayed for two hours.
Use since for a start point continuing to now.	We have lived here since 2010.
Use during for something that happens within a period.	He was quiet during the meeting.

Some time expressions need no preposition because the words themselves already function as time markers. Today, tomorrow, tonight, next week, and last year commonly appear on their own. These forms often replace a preposition plus noun structure when the time reference is already clear.

Idea	Example
Use no preposition with today and tomorrow.	I am busy today.
Use no preposition with next and last time expressions.	She leaves next week.
Use no preposition with tonight and similar adverbial time words.	We will call tonight.

In marks larger periods, on marks days and dates, and at marks exact times or points. By sets a latest time, for gives duration, since marks a starting point that continues, during places an event inside a period, and until or till extends action up to a limit. Zero preposition forms such as today, tomorrow, next week, and last year complete the system by showing that some time expressions stand alone without a preposition.