Raksha Bandhan, the Shravana Full Moon, and the 3rd House

Raksha Bandhan is observed on the Purnima of the lunar month Shravana, the tithi when the Moon stands opposite the Sun. Purnima is one of thirty tithis. It is not a solar date, so the civil calendar day shifts every year and must be taken from the place where the vow is kept. If the full-moon tithi starts or ends near sunrise, local custom decides which civil day holds the festival. A copied date from another city can be one day off, because the tithi boundary follows the Moon, not midnight.

In the kundli the 3rd house is younger siblings, courage, and the work of the hands. The thread ceremony is a custom of protection between brothers and sisters. It does not create a 3rd house that the chart lacks, and it does not repair a 3rd lord sitting in the 6th, 8th, or 12th. Mars is the karaka of siblings. If the question is about a brother's or sister's role in the life, read the 3rd lord and Mars, then the dasha that is running. The cotton or silk on the wrist is not that lord.

Households tie a protected thread, mark the forehead, and offer something sweet. Those gestures belong to the vow. They are not a graha placement and they do not change a mahadasha. People sometimes ask for a muhurta inside Purnima. The useful limits are ordinary ones: stay inside the tithi at that city, and do not treat a convenient evening as holier than the tithi itself. Bhadra, when it overlaps the full moon, is avoided by many families for the tying. That is a panchang caution, not a new yoga. The chart you already have is still read from the lagna and the Moon's nakshatra.