Marks Uttarayana - the sun's northward journey, the deva-yana (path of the devas). Considered the SUPREMELY auspicious 6-month half-year. Bhishma waited for Uttarayana to die. Pitri-tarpana + sun-worship + til-laddu offerings.
Why this date - the science
Astronomically, this is when the sun crosses the Tropic of Capricorn - the shortest day passes + longer days return. The first measurable lengthening of daylight. Modern science calls this winter solstice + the start of incrementally-warmer northern-hemisphere days.
Ayurvedic wisdom
Til (sesame) + jaggery = the supreme winter-warming food. Sesame oil is heating; jaggery breaks down phlegm + boosts iron. Eating til-laddu + til-gud in mid-January exactly when the body craves heat = perfect dietary timing. Also: bathing in cold water (cold-shock therapy) + applying til-oil before bath = the supreme winter abhyanga.
For the family & mind
Community festival. Kite-flying (Gujarat) + sugarcane (Punjab Lohri) + cow worship (Tamil Pongal) - all involve OUTDOOR family gathering in mild winter sun. Vitamin D synthesis + group-laughter + harvest-celebration = exactly the cure for late-winter depression. The festival HEALS seasonal-affective-disorder before modern medicine had a name for it.
Go deeper - every Hindu festival aligns spiritual, astronomical, ayurvedic and emotional layers on the same day. Read the full festival science explainer, check today's Panchang, or see the full festival calendar.