Month.so

First and last day of a month

Find the boundary dates and weekdays of any calendar month.

Build a month calendar while you read

Choose a month and year to inspect its exact calendar, weekday totals, weekends and boundary weekdays.

Building the calendar…

Total days
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Weekdays
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Weekend days
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First / last weekday
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The calculation runs in your browser.

First-day calculation

Create a date from the selected year, month and day 1. Its weekday determines the placement of the entire month grid.

Last-day calculation

Create the first day of the next month and subtract one calendar day. This works for every month, including December and leap-year February.

Why boundaries matter

Month boundaries drive reporting windows, subscriptions, accruals and recurring tasks. State the time zone when a boundary represents an exact instant.

A reliable method

Find the boundary dates and weekdays of any calendar month. A dependable answer starts by defining the reference and convention before doing the arithmetic.

  1. Define the reference

    Use one named month in one specific Gregorian calendar year. Write it down when another person must be able to reproduce the answer.

  2. Check the inputs

    Select both the month and year; February and every weekday layout depend on the year.

  3. Apply the calculation

    Find the first day of the month and the first day of the next month, then derive length and weekday positions from those boundaries.

  4. Label the result

    Use total days for duration, weekday counts for capacity and the calendar grid for recurring-date decisions.

Worked example

February 2028 has 29 days because 2028 is divisible by four and is not an excluded century year. Its weekday and weekend totals depend on which weekday February 1 occupies.

How to interpret the answer

For first and last day of a month, the useful answer includes its assumptions. Use total days for duration, weekday counts for capacity and the calendar grid for recurring-date decisions.

Accuracy checklist

  • Use a complete date whenever the result can cross midnight or a calendar boundary.
  • State the time zone or local-time basis instead of leaving it implied.
  • Label inclusive, exclusive, elapsed-time or calendar-based counting.
  • Recheck future results when civil-time or organizational rules may change.

Common mistakes

  • Using a familiar shortcut without checking whether it matches the guide's calendar convention.
  • A month name without a year is insufficient for weekday questions, and business-day totals also require a regional holiday calendar.
  • Sharing only the final number without the reference date, zone, unit or rounding rule.

Frequently asked questions

What is the quickest practical way to use first and last day of a month?

Start with one named month in one specific Gregorian calendar year. Use the interactive tool on this page, then check the result against the method and limitations below.

What can make a selected months result inaccurate?

A month name without a year is insufficient for weekday questions, and business-day totals also require a regional holiday calendar.

Does the calculator send my entries to the server?

No. The interactive calculation runs in the browser. For a result you plan to share, record the relevant date, time zone and counting convention alongside it.

Related selected months guides