"Take vengeance for the Children of Israel against the Midianites; afterward, you shall be gathered to your people." (31:2)
The Midrash Tanchuma, Mattos 2 on this Posuk asks: How many Shofar blasts is a person obligated to sound on Erev Shabbos? Three blasts." The obvious question: What is the connection between taking vengeance against Midyan with blowing the Shofar on Friday afternoon?
Rashi explains why Hashem commanded Israel to take revenge against Midyan, but not against Moav who also participated in causing Israel to sin? Rashi explains that Moav was spared because two "noble souls" were destined to emerge from them in the future: Rus and Naamah, from whom King David and Moshiach descend. However, this raises a question: The Gemarra Yevamos 47a states that "Converts are difficult for Israel." If converts are viewed so harshly, why would the nation of Moab be spared for the sake of two future converts?
The answer is: Moav was spared because of the spiritual bringing of holiness into the Jewish nation through their conversion. Now, the connection between the Midrash's question and the verse becomes perfectly clear: Why did Hashem say to take revenge only against Midyan and not Moav? Because of the converts Ruth and Naamah, in order to bring out of them Holy souls.
Why do we blow the Shofar on Erev Shabbos? The halachic reason for these blasts is to signal the community to cease their weekday work early, thereby fulfilling the commandment to add from the routine week onto the Heilige Sabbaos, Mosef mi-chol al Ha-kodesh. The Midrash purposefully linked these two topics to teach us a concept. Both the sparing of Moav for the sake of future converts and the Shofar Friday afternoon share the exact underlying spiritual mechanism: elevating the routine and attaching it to the Holy. (Kesef Nivchar Frankfurt 1712)
