Accuracy first — every position comes from DE440, NASA JPL's most precise planetary ephemeris.
Accuracy first — every position comes from DE440, NASA JPL's most precise planetary ephemeris.
Two calendars, both scanning ahead for days when a weekday meets the right nakshatra. One is strict and rare; the other comes round most weeks.
A yoga here is a combination — a weekday meeting a particular nakshatra, and nothing else. No birth details, no tithi, no planets. When the two named in the rule fall together, the yoga is held to run for as long as that nakshatra lasts, beginning at sunrise or when the nakshatra begins, whichever is later. That is the whole mechanism, and it is why these can be published as a calendar months ahead while a personal muhurat cannot.
The word does two jobs on this site and it is worth separating them. The Panchang's fourth limb is also called Yoga: one of twenty-seven, computed from the Sun's and the Moon's positions added together, and it changes roughly once a day whether it is auspicious or not. The yogas on this page are weekday-and-nakshatra combinations with names of their own. A day can carry an inauspicious Panchang yoga and an Amrit Siddhi Yoga at once; the two are answering different questions and neither cancels the other.
The dates and the windows are arithmetic. Which nakshatra the Moon is in, when it begins and ends, when your sun rose — these are computed from your date and place, and any correct source working from the same sunrise will agree. That such a combination is auspicious is the tradition's reading of it, long held and widely held, and still a reading rather than a measurement. A calendar can tell you when the combination occurs; what you make of it is not arithmetic.
Amrit Siddhi Yoga allows exactly one nakshatra per weekday — seven pairs in all — so it falls about two dozen times a year and is the one you wait for. Sarvartha Siddhi allows three to seven per weekday, thirty-four pairs, so it comes round several times a month and is the one you can usually find a workable date in. That frequency is the practical difference; both are read the same way.
These are general auspicious periods — on a given day they are the same for everyone in a city, because nothing in them depends on your birth details. A day chosen against your own chart is a different question, and it is counted from your birth star rather than from the calendar.