The day that happens twice

The Indian calendar counts a unit called a tithi. A tithi is a lunar day, but it is not a day of the clock. It is a stage in the widening gap between the moon and the sun, and it ends once that gap has grown by another 12 degrees.

The moon speeds up and slows down as it goes round us, so a tithi is not a fixed length. It can run to about 20 hours or stretch past 26. That is why it will not sit inside a calendar date.

There is a simple rule for handling this. Look at the moment of sunrise. Whichever tithi is running then gives the whole day its name.

Usually that works out neatly and the number goes up by one each morning. But suppose a tithi lasts longer than the gap between two sunrises, and suppose it starts shortly before one of them. Then the sun rises inside it, it runs all day and all night, and the sun rises inside it again. Two mornings, one number. The old word for this is vriddhi, meaning increase.

Here is the real case, from sheets computed for Delhi. The seventh lunar day of the bright fortnight began at 05:54 on Saturday 17 October 2026 and ended at 08:27 the next morning. Sunrise was 06:23 on the Saturday and 06:24 on the Sunday. Both mornings were the seventh.

You can spot a doubled day before it arrives. The number appears twice, and on the first of the two dates the sheet prints no ending time at all.

Then the household question: which of the two days do we keep for a fast or a birthday? The essay gives a test. Ask what hour your observance actually needs, and check both days for that hour. On the second date the tithi is usually finished by mid morning, so anything measured by midday, evening or moonrise belongs to the first. For a birthday kept by lunar day, the hour of birth decides.