One particular custom practiced on the day of Lag B'Omer is unique: Children go to parks and fields to play with bows and arrows. What is the reason for this peculiar custom? One well-known explanation has to do with the fact that during Rabbi Shimon's lifetime, no rainbow ever appeared in the sky. This is profoundly significant, because Genesis relates that the rainbow represented G-d's covenant never to destroy the world again even if the human race would degenerate to its status prior to the Flood. But as long as Rabbi Shimon was alive, his merit and piety alone were enough to ensure that G-d would not regret His creation, with no need for the rainbow. On the day of Rabbi Shimon's passing, however, the world was in need of the rainbow. Thus, each year on that day we recall this man's greatness by playing with the bow.
רבי חזקיה בשם רבי ירמיה כל ימיו של רבי שמעון בן יוחאי לא נראתה הקשת בענן.
Rabbi Chizkiyah said in the name of Rabbi Yirmiyah: During the lifetime of Rabbi Shimon bar Yochai, a rainbow was never seen in the clouds.
פני משה לירושלמי שם
הי' זכותו מגין על הדור ולא הי' צריך להראות הקשת בענן
Pnei Moshe on Yerushalmi
Rabbi Shimon's merit protected the generation, and there was no need for a rainbow.
Can the custom also be explained in a positive way?
לָא תְצַפֵּי לְרַגְלֵי דִמְשִׁיחָא עַד דְּיִתְחֲזֵי הַאי קֶשֶׁת בְּעָלְמָא (ד''א ל''ג בעננא) מִתְקַשְׁטָא בְּגַוְונִי נְהִירִין וְיִתְנְהִיר לְעָלְמָא. וּכְדֵין צְפֵי לֵיהּ לְמָשִׁיחַ.
Do not expect the footsteps of Moshiach until you see a rainbow, decorated in many colors, shining upon the earth. Then, you can expect Moshiach.
לדרת עולם FOR GENERATIONS FOR EVER — The word לדרת “for generations” is written missing a letter vav (implying that the sign will be necessary only for such generations as are “missing’’ in faith) because there will be some generations which will require no sign since they were completely righteous, such as the generation of Hezekiah, king of Judah, and the generation of R. Simeon ben Yochai.
בחרבי ובקשתי that is, by means of his spiritual weapons: his wisdom and his prayer.
What is the difference between a sword and a bow & arrow?