We were walking home from a friend's house after lunch on Shavuot last year. It had been a blazingly hot day, a real Jerusalem sharav, but at one point we were sure we felt a slight drizzle. As we entered the courtyard to our apartment complex, we felt it again. Then we noticed them: a group of nine- to 11-year-olds huddled together in what I can only describe as a "scheming posture." In the center was one child with an enormous water pistol. That's when we remembered. The holiday of Shavuot as it's observed in Israel is also known as "Yom Hamayim" - Water Day. "Run for it!" I yelled as we scampered towards our apartment before a stream of water headed our way. We avoided any serious soakingâ€¦ this time. But the battle had only just begun. The doorbell rang. Two of eight-year-old Aviv's friends were outside. "Can we use your terrace?" one of them asked. Before I could think whether this was a good or a bad thing for the Jews, Aviv had already ushered them inside. Now, we live in an upstairs apartment that has several inside levels; the sought-after terrace is actually three stories above ground level, giving anyone standing on it an unparalleled strategic advantage over enemies in the courtyard below. It truly is the high ground in the battle for Yom Hamayim supremacy. Aviv and his friends surveyed the scene from the terrace, then headed downstairs to our kitchen where they raided our collection of plastic water bottles that were waiting for recycling. They filled up three then resumed their positions. When the first volley of water was launched, the hapless soldiers below didn't know what hit them. What are the origins of this uniquely Israeli holiday custom? No one I asked could give me a definitive answer and the Internet wasn't much help either. Perhaps it has something to do with the parting of the waters of the Red Sea as the Jews left Egypt in preparation for receiving the Torah, the main event Shavuot commemorates. Or maybe it's more related to the symbolism surrounding Moses, who was rescued from the waters of the Nile and raised in Pharaoh's palace. My friend Yuval claims it's originally a North African custom that was elevated in importance when the country's secular founders were trying to emphasize the agricultural nature of the holiday. Or maybe it's because Shavuot usually falls at the beginning of the summer and it's just plain hot. It wasn't long before the there was another knock on the door. This time it was 12-year-old Merav's friends. More recruits for the Blum brigade. They too headed for the kitchen, but they were more interested in our supply of plastic sandwich bags. "Can you tie this for me?" asked an 11-year-old named Daniella, holding a filled bag. She and her friend Dara were building a not insignificant stockpile of water bombs. After the 10th bag, I told them to hold off, there might be other kids coming who'd want. Which there wereâ€¦ in droves. Over the course of the next half hour, no fewer than two dozen pre-teens, most part of a loose collection of friends of Merav and Aviv but others complete strangers, entered our kitchen, refilled their bottles and guns or built their own bombs, and headed for the terrace. At one point, I don't think there was anyone even left in the courtyard. Naturally, all of this created no small amount of mess. Puddles of water formed around the kitchen sink and the water tap in the entry-level guest bathroom. A small river of mud and twigs snaked from the front door to the terrace. My wife, Jody, pulled me aside. "I think that's enough," she said. But the kid inside of me had other ideas. "Why don't we just let them have fun?" I asked Jody. "Yom Hamayim is only once a year." Jody's eyes surveyed the accumulating devastation that was taking over our living room. "Don't worry," I said. "I'll be responsible for cleaning up. Just sit back and enjoy." "I think I'll enjoy it more if I don't look," Jody said with a smile and promptly closed herself off in a secure room while the Yom Hamayim battle continued unabated outside. For the next hour, I helped the combatants keep the supply lines open. I made sure no one slipped or got hurt. I cheered on the battle - especially when the target was the 12-year-old boys from Merav's class. We provided drinks and cut up watermelon. Eventually the battle wound down. The plastic bag supply ran out. Several girls were wrapped in towels as they shivered. I actually managed to get a few kids - led by Aviv, Merav, Dara and Daniella - to help clean up the garbage below. As I squeegeed the water towards the terrace drain, one of the kids asked me, her eyes glazed with drops of water and appreciation, "Is your house open like this every year?" "It is now," I replied. As Jody emerged from her room, I said "next year, we have to be better prepared. We need to stock up on plastic bags and save up the recycling bottles for several weeks. "Or maybe," Jody said, as she surveyed the damage, "We'll just lock the doors and pretend to not be home." The writer writes a regular blog at ThisNormalLife.com.