# Day of the Week Calculator

Figure out the day of the week for any date using our calculator below.

## Day of the Week:

Sunday

August 4, 2024 is a Sunday.

It is day number 217 of 2024,
and there are 150 days left.

It's the 1st Sunday in August,
and the 31st Sunday in 2024.
Learn how we calculated this below

## How to Find What Day of the Week It Is?

There are a few methods you can use to figure out what day of the week it is, with the easiest being the calculator above. Search engines, digital assistants, smartphones, tools like our current date calculator, and even web browsers are other great ways to find the current day using built-in calendars.

You might be surprised to find out that you can actually calculate the day of the week for any date using a formula. The formula is called Zeller’s congruence, and it is an algorithm created in the 19th century to calculate the day of the week for any date in the Julian or Gregorian calendar.

### Zeller’s Congruence Day of the Week Formula

Zeller’s congruence for the Gregorian calendar (the calendar that is most widely in use today) defines the following formula to find the day of the week for a date:

Day = \left [ D+ \left \lfloor \frac{13(M+1)}{5} \right \rfloor + Y + \left \lfloor \frac{Y}{4} \right \rfloor + \left \lfloor \frac{Z}{4} \right \rfloor +5Z \right ] (\text{mod 7})
[formula may scroll beyond screen]

Where:
D = day of the month
M = adjusted month number (3 = March, 4 = April, 5 = May, …,13 = January, 14 = February)
Y = year of the century (year mod 100)
Z = zero-based century (⌊year / 100⌋)

The result of the formula is the day of the week where 0 = Saturday, 1 = Sunday, 2 = Monday, 3 = Tuesday, 4 = Wednesday, 5 = Thursday, & 6 = Friday.

When applying Zellner’s congruence, it’s important to note that the months of January and February are treated as months 13 & 14 of the previous year.

For example, let’s calculate the day of the week for July 20, 1969 using Zellner’s congruence.

Let’s start by calculating the key values for the formula.

D = 20
M = July = 7
Y = 1969 % 100 = 69
Z = ⌊1969 / 100⌋ = 19

Day = \left [ 20+ \left \lfloor \frac{13(7+1)}{5} \right \rfloor + 69 + \left \lfloor \frac{69}{4} \right \rfloor + \left \lfloor \frac{19}{4} \right \rfloor +(5 \times 19) \right ] (\text{mod 7})
[formula may scroll beyond screen]
Day = \left [ 20+20+69+17+4+95 \right ] (\text{mod 7})
[formula may scroll beyond screen]
Day = \text{225 mod 7}
Day = 1

Recall from above that day 1 is equal to Sunday in Zellner’s congruence; thus, the day of the week of July 20, 1969 was Sunday.

## Days of the Week

The seven-day week has been used for millennia by various cultures, with each day named after celestial bodies or mythological figures.

• Sunday: Named after the Sun, this day is considered the first day of the week according to the international standard ISO 8601, but in many cultures, it’s regarded as the last.
• Monday: Named after the Moon, it’s the second day of the week.
• Tuesday: Comes from the Old English “Tiwesdæg,” named after Tiw or Tyr, the Norse god of war. It’s the third day of the week.
• Wednesday: Named after Woden or Odin, the chief Norse god, its name comes from “Woden’s day,” marking the fourth day of the week.
• Thursday: Named after Thor, the Norse god of thunder, “Thor’s day” is the fifth day of the week.
• Friday: Comes from “Frigg’s day,” named after Frigg, the Norse goddess of love and the sky, marking the sixth day.
• Saturday: Named after Saturn, the Roman god of wealth and agriculture, this is the seventh day of the week.

## What is the First Day of the Week?

The first day of the week varies across different cultures, calendars, and even within the context of international standards.

According to the international standard ISO 8601, Monday is designated as the first day of the week.[1] This standard is widely adopted in many parts of the world, especially in European and Asian countries, where the working week typically runs from Monday to Friday, with Saturday and Sunday as the weekend.

However, in several other cultures and calendars, Sunday is traditionally considered the first day of the week, reflecting religious and historical practices. For instance, in the United States, Canada, and Japan, calendars typically display Sunday at the start of the week, aligning with Judeo-Christian traditions wherein Sunday is observed as the Lord’s Day or the Sabbath in some denominations.

You might be interested in using our business day calculator to find how many weekdays there are between dates.

## References

1. International Organization for Standardization, ISO 8601-1:2019(en) Date and time — Representations for information interchange — Part 1: Basic rules, https://www.iso.org/obp/ui/#iso:std:iso:8601:-1:ed-1:v1:en:term:3.1.2.16