This site describes solve-engine as it is on main: 2.43.0, which npm does not have yet. npm installs 2.40.0, so a page may show an answer that version does not give yet.
Weekdays & week numbers
Package:
DATETIME_PACKAGE. Registered bycreateEngine(); for a slimmer engine, register it explicitly (see choosing packages).
A date names a day on the calendar, and three things about it are often wanted on their own: the day of the week it falls on, its month, and its week number, the count of weeks from the start of the year that offices and planners use to name a week (“week 12”). Each can be asked as a question, about a date, about today, or about a day some time from now.
The questions about today are worked out for one fixed moment, noon on Wednesday 11 March 2026 in London, so that they can be checked; the notepad under each example works them out for your own today.
The day of the week
Section titled “The day of the week”what day is it on, day of the week on and weekday on all ask which day of
the week a date falls on (day of week on too, without the the), and so do
the same words with of in place of on (weekday of, day of the week of,
day of week of), as month of asks for the month below. The answer is the day’s name.
what day is it on 2026-12-25 // Fridaywhat day is it on 25 december // Fridayday of the week on 1 January 2000 // Saturdayweekday on 2026-12-25 // Fridayweekday of 2026-12-25 // Fridayday of the week of 1 January 2000 // Saturdayday of week of 2026-12-25 // FridayEach reads the same in a function’s formula, so a function can name the day of whatever date it is given:
day(d) = weekday of dday(2026-10-01) // Thursdayday(2026-12-25) // Fridaywhat day is it on its own asks about today, and a length of time after in
asks about a day that far ahead. what day will it be reads the same, with in
or on.
what day is it // Wednesdaywhat day is it in 3 days // Saturdaywhat day will it be in 3 days // Saturdaywhat day will it be on 2026-12-25 // FridayThe month
Section titled “The month”The month questions follow the same shape: what month is it for today, on a
date, in a length of time, and month of a date.
what month is it // Marchwhat month is it on 2026-12-25 // Decemberwhat month is it in 3 months // Junewhat month will it be in 10 months // Januarymonth of 2026-12-25 // DecemberThe week number
Section titled “The week number”The week number here is the ISO week, the international standard (ISO 8601) most of Europe and most planning software use. Its weeks run Monday to Sunday, and week 1 is the week holding the year’s first Thursday. So the last days of December can fall in week 1 of the next year, and the first days of January in week 52 or 53 of the one before: Friday 1 January 2027 is in week 53 of 2026.
week number of a date gives its week, and week number alone gives this
week’s. what week is it asks the same with on a date or in a length of
time.
week number // 11week number of 2026-12-25 // 52week number on 2026-12-25 // 52week number of 2027-01-01 // 53what week is it // 11what week is it on 2026-12-25 // 52what week is it in 2 weeks // 13what week will it be in 2 weeks // 13As a conversion
Section titled “As a conversion”The same three answers can be asked for with as, the way a date is shown in
another form, which reads well after a date worked out on the same line.
2026-12-25 as weekday // Friday2026-12-25 as month // December2026-12-25 as week // 52The day and month names are shown in the reader’s language when the app
showing the notes is set to one: 2026-12-25 as weekday shows Freitag to a
reader whose app is set to German. The answer is still the English name
underneath, so a check such as (2026-12-25 as weekday) == "Friday" holds in
every language. How an app chooses the language is covered in
formatting results.
The boundary
Section titled “The boundary”- The week number is always the ISO one. The other conventions (weeks starting on a Sunday, or week 1 as the week holding 1 January, as in the United States) are not read.
- A day name on its own is not a date, so
Tuesday is a business dayis refused rather than guessed; whether a date is a working day is asked of a date, on working days. - The answer is the name or the number alone. To show a date’s full form, see displaying dates.