The Calendar
What the atlas records for this date, across the traditions it covers.
Observed On This Date
Martyrdom remembered
10 Muharram
Tabular reckoning; a local sighting may move it by a day.
This Week In Mathematics History
Dates for the Hebrew and Islamic calendars are the tabular reckoning, which is what a printed calendar prints. Communities that begin a month on a sighted crescent may differ by a day, and differ from one another by place.