Weekday Calculator
Determine the weekday for any date — past or future. Count business days between two dates, exclude holidays, and find what day a deadline falls on.
Enter your values above to see the results.
Tips & Notes
- ✓Business day deadlines count Monday through Friday excluding holidays. A 30-business-day deadline spans approximately 42 calendar days — nearly 6 full weeks, not 30 days.
- ✓When a deadline falls on a weekend or holiday, most legal contexts require advancing to the next business day. Confirm your jurisdiction rule — some require the prior business day for filings.
- ✓The Doomsday algorithm lets you calculate any day of the week mentally: memorize anchor dates (4/4, 6/6, 8/8, 10/10, 12/12) and the current year anchor day, then count forward or backward.
- ✓Payroll and invoice due dates that fall on weekends are typically processed the preceding Friday — verify your payroll provider policy to avoid delayed payments.
- ✓For international contracts, confirm which country calendar applies for holiday exclusions — US Labor Day, UK Bank Holidays, and UAE National Day all affect different parties differently.
Common Mistakes
- ✗Assuming 5 business days always equals 7 calendar days — it equals 7 only when no holidays fall in the span. Holiday weeks stretch the count to 8 or more calendar days.
- ✗Not specifying the holiday set when counting business days — a 5-business-day calculation including Christmas produces a different calendar date than one in August.
- ✗Forgetting that 2100 is not a leap year — the Gregorian calendar skips leap year on century years not divisible by 400. 1900 was not, 2000 was, 2100 will not be.
- ✗Confusing inclusive and exclusive deadline counting — 10 days from Monday can mean Wednesday (exclusive, 10 full days later) or Thursday (counting Monday as Day 1).
- ✗Using Sunday as the start of the work week in jurisdictions where Monday is standard — ISO 8601 defines weeks starting Monday, but many US systems use Sunday, affecting week number calculations.
Weekday Calculator Overview
A weekday calculator determines what day of the week any date falls on — past or future — and counts business days between dates by excluding weekends and specified holidays. These calculations appear in contract deadlines, project scheduling, payroll processing, court filings, and any planning where specific weekdays matter.
Finding the day of the week — Doomsday anchor method:
Key anchor dates in any year always fall on the same weekday: 4/4, 6/6, 8/8, 10/10, 12/12, and the last day of February
EX: What day is October 25, 2025? The anchor day for 2025 is Friday (4/4/2025 = Friday). October 10 = Friday. Count from Oct 10 to Oct 25 = +15 days = +2 weeks + 1 day. Oct 25 = Saturday.Business days in each month by week configuration:
| Month Length | Minimum Business Days | Maximum Business Days | Typical Range |
|---|---|---|---|
| 28 days (Feb, non-leap) | 20 | 20 | Always 20 |
| 29 days (Feb, leap year) | 20 | 21 | 20 or 21 |
| 30 days | 20 | 23 | 21–22 |
| 31 days | 21 | 23 | 22–23 |
| Deadline Type | Calendar Days | Approx. Business Days | Key Distinction |
|---|---|---|---|
| 30-day contract term | 30 | ~21 | Calendar days include weekends |
| 30 business days | ~42 | 30 | 6 calendar weeks |
| 90-day notice period | 90 | ~65 | Quarters vary by starting month |
| Notice to quit (14 days) | 14 | 10 | Most residential tenancy law uses calendar days |
| Court filing (10 business days) | ~14 | 10 | Excludes weekends and court holidays |
Frequently Asked Questions
Enter any date into the calculator and it instantly computes the weekday using a mathematical algorithm (Zeller's congruence). For a quick mental method: January 1, 2025 is Wednesday. Each non-leap year shifts the calendar forward 1 weekday; each leap year shifts 2 weekdays. Count years and shifts from a known anchor. Example: 2026 = 2025 + 1 day → Thursday January 1, 2026.
Count the total calendar days between the two dates, then find the remainder after dividing by 7. Each remainder value maps to a weekday shift. For business days only: subtract 2 days for each complete week (5 business days per week = 7 calendar days), then handle the remaining partial week manually. The calculator performs this automatically and optionally excludes weekends.
Add exactly 30 calendar days — not one month. From January 15: January has 31 days, so 31 − 15 = 16 remaining days in January. Then 30 − 16 = 14 days into February → February 14. One month would be February 15 — a different date. This distinction matters in contracts: 30 days from January 15 is February 14; one month from January 15 is February 15.
A standard US calendar year has approximately 260 to 262 business days: 365 calendar days minus 104 weekend days (52 Saturdays + 52 Sundays) minus federal holidays (typically 10 to 11 days). Leap years add 1 calendar day for 261 to 263 business days. The exact count depends on which holidays fall on weekdays — holidays landing on weekends are observed on Friday or Monday, affecting the total.
ISO 8601 defines weeks as Monday through Sunday, with week 1 being the week containing the first Thursday of the year. This means January 1 can fall in week 52 or 53 of the previous year. Example: January 1, 2021 was a Friday — Thursday of that week was December 31, 2020, so January 1 belonged to week 53 of 2020. US weeks traditionally start Sunday, giving different week numbers for the same date.
Each common year (365 days = 52 weeks + 1 day) shifts the calendar forward 1 weekday. Each leap year shifts 2 weekdays. Example: January 1, 2024 was Monday (2024 is a leap year). January 1, 2025 = Monday + 2 = Wednesday. January 1, 2026 = Wednesday + 1 = Thursday. This pattern repeats with slight variation every 400 years, the length of the full Gregorian calendar cycle.