David Natan Lesser was a correspondent of the Rebbe over many years. No reliable published biography is available; the summary below is drawn from the letters themselves.
A recurring thread in the Rebbe's twelve letters to him is Chanukah. In several years the Rebbe acknowledged Chanukah candles he had received from him, and drew lessons from the festival: the miracle of the oil and levels of mesirus nefesh (IK 1316), inspiring action through Chassidus (IK 1942), rising like the shamash that lights the others (IK 2316), spreading the wellsprings and the eight princes (IK 4971), the eternal light of the redemption (IK 6575), transforming darkness into light in the month of Teves (IK 6619), and spreading the inner light of Chassidus (IK 8559).
Other letters take up questions he raised on tzitzis and the Hebrew letters: halachic opinions on machine-made tzitzis, and not publicizing stringencies as binding (IK 1722); the merit of the mitzvah of tzitzis (IK 3578); caution about printing the alef-beis with tagin (IK 2160); and teaching children Ksav Ashuri, and its practice in Jewish communities (IK 2398). One letter discusses Chai Elul and the meaning of Elul in avodah (IK 3777).
See also: Shabbos and holidays – Chanukah; Mitzvos in daily life – Tefillin, Rabbeinu Tam and tzitzis.