In addition to a shop, there is a small, square field next to the shop. If the player ventures to the square between Monday night at 8:00 pm and Tuesday morning at 4:00 am, the player will see two Clefairy dancing around a rock. Once the player enters the square, the Clefairy will be startled and run away, leaving behind Kanto's only smashable rock in the Generation II games, which hides a Moon Stone. However, if the player leaves the area (or enters the shop) before smashing the rock, the rock will disappear and there will be no way to get it back until the following Monday.
In Pokémon HeartGold and SoulSilver, there will be four Clefairy dancing around a pond with the moon's reflection, with two Clefairy standing aside watching. During the dance, a Clefairy will see the player and all the Clefairy will run away, leaving behind an item ball with a Moon Stone inside.
I assume this is what you mean? OR if you're looking for where to catch it, you can find it in the Safari Zone or Mt. Moon at any time.