Two independent cycles · one calendar moment

When tithi and vāra meet

A tithi follows the changing angle between the Moon and Sun. A vāra follows the seven-day week. The two counts run on separate schedules, so a calendar rule may look for a particular member of each cycle in the same local period.

Two clocks running together

The cycles are independent, but a rule can intersect them.

Tithi30 Moon–Sun angle steps

Vāra7 weekdays

The pointers move at different illustrative speeds to make the changing relationship visible. A real tithi has variable duration, so this simplified diagram illustrates the relationship rather than timing it.

Tithi is a lunar-angle classification.

Thirty tithis divide one complete Moon–Sun elongation cycle. Their boundaries move through civil-clock time independently of the weekday sequence.

Vāra is the weekday classification.

The familiar sequence from Ravivāra through Śanivāra repeats every seven days, independently of the Moon–Sun angle.

Sunday timing example

Three terms can combine to describe an observance’s timing.

Ravivāra is Sunday. When Pradoṣa vrata occurs on that weekday, Ravivāra describes when the recurring observance takes place; it does not create a separate observance.

tithiTrayodaśīvāraRavivāracontrolling timepradoṣasame observancePradoṣa vrata on Sunday

The example combines three conditions: Trayodaśī identifies the tithi, Ravivāra identifies the weekday, and pradoṣa identifies the controlling evening period.Pradoṣa vrata is a twice-monthly Śaiva observance held during pradoṣa, the early-evening period. For the calendar rule used here, Trayodaśī must overlap that period. When that overlap occurs on Ravivāra, it is the Sunday occurrence of the same Pradoṣa vrata.

References: Sewell and Dīkṣita on tithi