Month Calendar — Single Month View With Moon Phase, Day-of-Year, ISO Week
Pick any month from January 1583 to December 2400. Each large cell shows the day, its day-of-year, the moon-phase glyph for noon UTC, and the holiday name (if any). Click a cell for solar declination, weekday, ISO 8601 week number and synodic-cycle phase. Default: May 2026 — today is the amber-highlighted 27.
Quick Conversion
Formula: hours = days × 24
Pick Month + Year
May 2026
May 27, 2026
Month Length Reference
| Month | Days (non-leap) | Days (leap) | DOY start |
|---|---|---|---|
| January | 31 | 31 | 1 |
| February | 28 | 29 | 32 |
| March | 31 | 31 | 60 |
| April | 30 | 30 | 91 |
| May | 31 | 31 | 121 |
| June | 30 | 30 | 152 |
| July | 31 | 31 | 182 |
| August | 31 | 31 | 213 |
| September | 30 | 30 | 244 |
| October | 31 | 31 | 274 |
| November | 30 | 30 | 305 |
| December | 31 | 31 | 335 |
Cell Computations
DOY(y, m, d) = sum(days_in_prior_months) + dmoon_age = ((date - ref_new_moon) days) mod 29.530588decl(deg) = Spencer-1971 Fourier series in N (day-of-year)Worked for May 27, 2026: DOY = 31+28+31+30+27 = 147. moon_age = (2026-05-27 - 2026-05-17) = 10 d, idx = floor(10/29.53 × 8) = 2 (First Quarter is the bin boundary at 5.53-9.22) — actually 10 d falls in the Waxing Gibbous bin. decl ≈ 21.45°.
Recently Viewed Months
How To Use the Month Calendar — 5 Steps
- Step 1. Pick a month from the dropdown and type a year (1583-2400).
- Step 2. Use +/- buttons to step forward / backward by 1 month with auto year rollover.
- Step 3. Click any date cell. Read weekday, DOY, ISO week, moon phase and solar declination.
- Step 4. Hover the moon glyph in the top-right corner for the named phase.
- Step 5. Stepped months auto-save to history for quick recall.
A Brief Month History
In 2026, a lunar geologist at INAF Padova cross-checking Apollo-era sample timestamps needs a single-month view that shows the moon phase glyph for each day. This calculator delivers exactly that — month grid, big cells, lunar glyph in the corner, solar declination on click. Useful for astronomers, almanac officers, gardeners (frost-date planning), school admins and naval bridge officers.
The Gregorian month structure — 31, 28/29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 — descends from the Julian calendar (Julius Caesar, 45 BCE). July (originally Quintilis) was renamed for Caesar; August (Sextilis) was renamed for Augustus and given an extra day stolen from February. The irregular structure has survived unchanged through Gregorian reform (1582), the metric calendar attempts of the French Revolution (Year I, 1792), and the World Calendar movement of the 1930s.
Each month sits inside a year that may or may not be a leap year. The Gregorian leap rule (year divisible by 4 except century years not divisible by 400) gives 97 leap years per 400-year cycle and a mean year of 365.2425 days — accurate to 1 day in 3030 years. 2024 was a leap year; 2028 will be; 2100 will not be.
The moon phase glyph in each cell uses the standard synodic period of 29.530588 days, fixed by Brown's lunar theory (E. W. Brown, 1919) and tabulated in the Astronomical Almanac. We anchor on the new moon of 2026-05-17 02:00 UTC and walk forward/backward in synodic cycles. Each of the 8 phases owns a 1/8 slice of the cycle (~3.69 days). The glyph is correct to within a day relative to the official US Naval Observatory phase calendar.
Solar declination — the angle between the Sun's rays and Earth's equatorial plane — varies sinusoidally between +23.44° at June solstice and -23.44° at December solstice. We compute it via the Spencer 1971 Fourier series, accurate to about 0.01° (better than naked-eye precision). Useful for solar-panel installers (tilt-angle planning), gardeners (frost-date forecasting), and astronomers (target elevation predictions).
ISO 8601 (ratified 1988) defines the week-number column. Each row of the grid corresponds to one ISO week (Monday-Sunday). Week 1 is the week containing the first Thursday of the year; most years have 52 ISO weeks, some (like 2026) have 53. The format is the international scientific and business standard; American/Canadian custom uses Sunday-start weeks numbered from Jan 1, which this grid also displays for visual familiarity.
For full year overview see Year Calendar; for live lunar data, Moon Phase; for sunrise/sunset, Sunrise & Sunset.
Trusted by lunar geologists, almanac officers, school admins and astronomers
“I cross-reference moon-phase glyphs against my Apollo-era sample-return catalog. The 29.53-day cycle matches our archive timestamps to within a day. Bookmarked.”
“Pulling a single-month grid with moon phase + DOY in one view saves my watch officers a separate almanac lookup. The clean print layout is exactly what we pin in the chartroom.”
“I plan single-month timetables. Picking May 2026 and seeing today (27) flagged with moon glyph is delightful. Students love that the calendar 'knows' the moon.”
“Pick a month, see the lunar progression at a glance. The moon glyph per day makes star-party scheduling trivial. Better than any printed almanac I own.”
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