The 3rd of Tamuz is the date we mark the yahrzeit of the Lubavitcher Rebbe.
It is said that before the Six Day War, the Rebbe initiated an operation of having every male Jew put on tefillin (phylacteries).
Later, when asked why he chose this specific commandment, the Rebbe gave two answers:
According to the Talmud, from the moment a Jew puts tefillin on his head – even if it’s only once in his lifetime – he is uplifted to a higher level in his Judaism.
“When a Jew in Miami sees photographs of Jews at the Western Wall putting on tefillin, he feels an urge to put on tefillin himself,” explained the Rebbe, as the second reason for choosing this commandment.