Radak: It is to tell the place of the dance, which was outside Shilo and near the vineyards. The told the men to hide in the vineyards (verse 20). It was "mi'Yamim Yamimah" (annual); Targum Yonasan says, from Mo'ed to Mo'ed. Perhaps it was every Pesach, or every Sukos, which is the time of Simchah, or Yom Kipur, on which the girls used to dance in the vineyards (Ta'anis 26b).