The Miracle of the Sun standing still was only in order to allow Israel to defeat the enemy by their own power in a natural way.
Gimmel Tammuz is the day that the Previous Lubavitcher Rebbe was released from Soviet prison in 5687 (1927).
Prior to that, we find that it was on this day that Yehoshua bin Nun performed a tremendous miracle, causing the sun to stand still. The story behind the miracle is that Israel were doing battle with the enemy. Sunset was approaching, and with the setting of the sun the battle would cease and the enemy would be able to escape. In order to enable Israel to defeat the enemy, Yehoshua instructed the sun to “be silent”, causing it to cease it’s flight in the heavens, preserving the daylight so that the battle could be won.
The Rebbe asks an obvious question: if one has the power to cause the sun (and the entire heavenly system with it) to pause, why not simply bring about the defeat of the enemy in a direct fashion? The preceding verses in Sefer Yehoshua tell us that more of the enemy were killed by stones which fell upon them from heaven than were killed by the sword. Is it not a simpler matter to rain down some more stones than to freeze the entire heavenly system?
The answer the Rebbe gives is that the battle is Israel’s battle, it must be won by them under their own power. Of course Hashem is helping with miraculous assistance from above, but this is only assistance. The war must be fought and pursued by Israel in a natural fashion, even if on top of that there is super-natural assistance. Thus, instead of Yehoshua simply bringing about the defeat of the enemy and Israel stand by passively, he used his power to give Israel more time to overcome the enemy in a natural way.
In our day, Gimmel Tammuz is primarily associated with the transition from the years when we saw the Rebbe to the current situation where the Rebbe is not seen except in videos and dreams. Moshe Rabbeinu is likened to the sun, and the Rebbe, Moshe Rabbeinu in our generation, is still providing all the assistance necessary to win the war against evil. But the war must be won by us, under our own power. While the Rebbe was visible to all, it was possible to “rely on the Rebbe” to win the war. Today, when it seems that the sun, Moshe Rabbeinu, has “stopped moving” it is only in order to allow us to overcome the enemy using natural means.
For a more detailed explanation, with comprehensive references from Chassidus, see the Kuntres Inyonei Moshiach and Geulah for Gimmel Tammuz.