星期几计算器
输入任意日期，即时查询该日期是星期几。本计算器适用于历史、当前和未来日期，还可显示该日期是一年中的第几天和 ISO 周数。
Result
Thursday
April 2, 2026
常见问题
2000 年 1 月 1 日是星期几？
2000 年 1 月 1 日是星期六。你可以在星期几计算器中输入 2000-01-01 进行验证。
2026 年 7 月 4 日是星期几？
2026 年 7 月 4 日是星期六。独立日落在周六意味着美国联邦假日将在 2026 年 7 月 3 日（周五）补休。
如何查找某个日期是星期几？
在我们的星期几计算器中输入日期——它会即时显示工作日名称、是周末还是工作日、年内第几天（1-365/366）以及 ISO 周次。如需手动计算，可以使用泽勒公式（Zeller's Congruence）或末日算法（Doomsday Algorithm）。
什么是 ISO 周次？
ISO 周次（ISO 8601）标识一年中的特定周。周次从周一到周日，第 1 周定义为包含该年第一个周四的那一周。一年有 52 或 53 个 ISO 周。1 月初的几天可能属于上一年的最后一周。
什么是年内第几天？
年内第几天（也称序数日期）是从 1 到 365（闰年 366）的数字，表示日期在当年的位置。例如，2 月 1 日是第 32 天，普通年 12 月 31 日是第 365 天。
如何手动计算星期几？
使用泽勒公式：h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7，其中 q 是日，m 是月（1 月和 2 月视为上一年的第 13、14 个月），K 是年份的后两位，J 是世纪数。结果映射为 0=周六，1=周日，2=周一，以此类推。
2026 年圣诞节是星期几？
2026 年圣诞节（12 月 25 日）是周五。你可以使用星期几计算器查找任何未来或过去的日期。
此计算器能查找 1970 年以前的历史日期吗？
可以。该计算器支持浏览器日期选择器允许的任何日期，通常从公元 0001 年到 9999 年。它使用儒略历前推历法，将公历系统向 1582 年之前延伸——这是历史日期计算的标准方法。
What is the Day of the Week Calculator?
The Day of the Week Calculator instantly tells you what day of the week any date falls on — past, present, or future. Enter any date and the calculator returns the weekday name, whether it is a weekend or weekday, the day of the year (1–366), and the ISO week number. This is useful for scheduling, historical research, trivia, and any situation where you need to know the weekday for a specific date without consulting a paper calendar.
How to Find the Day of the Week for Any Date
Use the date picker at the top of the tool, or type a date directly into the input field. The calculator updates instantly as you change the date. To return to today's date, click the calendar icon next to the input.
- Click the date field and select or type the desired date.
- The large display shows the weekday name immediately.
- Below the large display, view the day index, day of year, and ISO week number.
- Click "Copy" to copy the full result to your clipboard.
How to Calculate the Day of the Week — Zeller's Congruence
Zeller's Congruence is a classic algorithm for computing the day of the week for any date in the Gregorian calendar. The formula for the Gregorian calendar is:
h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ - 2J) mod 7
Where:
- h — the day of the week (0 = Saturday, 1 = Sunday, 2 = Monday, …, 6 = Friday)
- q — the day of the month
- m — the month (3 = March, 4 = April, …, 14 = February; January and February are counted as months 13 and 14 of the previous year)
- K — the year of the century (year % 100)
- J — the zero-based century (⌊year / 100⌋)
For example, to find the day for July 4, 2026: q = 4, m = 7, K = 26, J = 20. Plugging in gives h = 0, which maps to Saturday — the correct answer.
Our calculator uses JavaScript's built-in Date object which implements the same Gregorian calendar arithmetic, producing accurate results for any date from year 1 through year 9999.
Understanding ISO Week Numbers
The ISO 8601 week number is the internationally standardised way to refer to a specific week within a year. Key rules:
- Weeks run Monday to Sunday.
- Week 1 is the week containing the first Thursday of the year (equivalently, the week containing January 4th).
- A year has either 52 or 53 ISO weeks. Years that start on Thursday, or leap years that start on Wednesday, have 53 weeks.
- Days in early January can belong to ISO Week 52 or 53 of the previous year, and days in late December can belong to Week 1 of the next year.
For example, January 1, 2026 is a Thursday, so it falls in ISO Week 1 of 2026. But January 1, 2016 was a Friday; since the first Thursday of 2016 was January 7th, January 1 belongs to ISO Week 53 of 2015.
Historical Calendar Facts
The Gregorian Calendar Reform (1582)
Pope Gregory XIII introduced the Gregorian calendar in October 1582 to correct accumulated drift in the Julian calendar. Ten days were skipped — October 4 was followed by October 15. Different countries adopted the reform at different times: Britain and its colonies switched in 1752, Russia not until 1918. This calculator uses the proleptic Gregorian calendar (extending backward before 1582), which is the standard for historical date calculations in most software.
The Seven-Day Week
The seven-day week has been in continuous use for over 2,000 years, tracing back to Babylonian astronomy and Jewish tradition. It was officially adopted by the Roman Empire in 321 AD when Emperor Constantine declared Sunday the first day of the week. The names of the days derive from classical planets and Norse gods: Sunday (Sun), Monday (Moon), Tuesday (Tiw/Mars), Wednesday (Woden/Mercury), Thursday (Thor/Jupiter), Friday (Frigg/Venus), and Saturday (Saturn).
Doomsday Algorithm
John Conway developed the Doomsday algorithmas a mental arithmetic trick to find the day of the week for any date. The algorithm exploits the fact that certain easy-to-remember dates always fall on the same day of the week within any given year — called the "Doomsday" for that year. For 2026, the Doomsday is Saturday. Memorizing a few anchor dates per century allows a trained person to calculate any day in their head within seconds.
Notable Historical Dates
| Date | Day | Event |
|---|---|---|
| July 4, 1776 | Thursday | US Declaration of Independence |
| November 11, 1918 | Monday | World War I armistice signed |
| July 20, 1969 | Sunday | Apollo 11 moon landing |
| January 1, 2000 | Saturday | Y2K / New Millennium |
| September 11, 2001 | Tuesday | 9/11 attacks |
| January 20, 2025 | Monday | US Presidential inauguration |
Day of the Year Reference
The "day of year" counts from January 1 (day 1) to December 31 (day 365, or day 366 in a leap year). This is sometimes called the ordinal date. Quick reference for the first day of each month in a non-leap year:
| Month | First Day of Month (day of year) |
|---|---|
| January | Day 1 |
| February | Day 32 |
| March | Day 60 |
| April | Day 91 |
| May | Day 121 |
| June | Day 152 |
| July | Day 182 |
| August | Day 213 |
| September | Day 244 |
| October | Day 274 |
| November | Day 305 |
| December | Day 335 |
In leap years, add 1 to all days from March 1 onwards.
Related Tools
- Date Calculator — Calculate the number of days between two dates or add/subtract days from a date
- Business Days Calculator — Count working days between two dates, excluding weekends
- Age Calculator — Calculate exact age in years, months, and days from a birthdate
- Countdown Calculator — Count down to a future event or date
Common Use Cases
- Verify the day of a historical event or birthday
- Determine if a future date falls on a weekend before scheduling
- Settle trivia disputes about what day a famous date occurred
- Check ISO week numbers for payroll, project management, or reporting
- Calculate the ordinal day of the year for scientific or logistical purposes
- Plan recurring weekly events by knowing what day a specific date lands on