Tithi calendar — the days families keep.

Most often the Smarta and Vaishnava difference. Where the eleventh tithi is touched by the tenth at sunrise, Vaishnava practice defers the fast by a day while Smarta practice does not — so the two traditions keep different dates deliberately. Follow your family’s or your temple’s practice; a calendar cannot know which tradition you belong to.

At moonrise, not at sunset — and this is the detail most often missed. Moonrise on a waning fourth is late, commonly after nine in the evening, and it varies by city more than any other timing here. Set your location above so the moonrise time is right for where you are.

Because most follow the lunar calendar. A lunar month is about twenty-nine and a half days, so twelve of them fall short of a solar year by roughly eleven days — which is why Diwali and Navratri shift each year, and why an extra month is inserted periodically to reconcile them. Solar observances such as Pongal barely move at all.

Yes, and both are normal. A tithi that begins and ends between two sunrises is skipped — kshaya tithi — and one that covers two sunrises is repeated, vriddhi tithi. Neither is inauspicious in itself; they are consequences of the tithi being between nineteen and twenty-six hours long while a day is twenty-four.

Most families keep the local date, since the observance rules depend on local sunrise, sunset and moonrise. A tithi begins at one moment worldwide but falls on different calendar dates by timezone. Set your city above and the dates adjust. Where a family keeps the Indian date by preference, that is also a settled practice and not wrong.

The 4th, 9th and 14th. Rikta means empty, and classical practice avoids them for beginnings — starting a journey, a business, or anything auspicious. They are not avoided for worship, which is why Sankashti, Navami and Shivaratri all fall on them. Worship is not a beginning, and none of these restrictions apply to ritual.

Depends what for. Pournami is the most generally auspicious for worship; Dashami and Tritiya are favoured for beginnings; Ekadashi is the most widely kept as a fast. For choosing a date for something specific, the tithi is one factor among several — the Muhurat Finder weighs them together.