וְהָיָה אֲשֶׁר לֹא יַעֲלֶה מֵאֵת מִשְׁפְּחוֹת הָאָרֶץ אֶל יְרוּשָׁלִַם לְהִשְׁתַּחֲוֹת לְמֶלֶךְ ה' צְבָקוֹת וְלֹא עֲלֵיהֶם יִהְיֶה הַגָּשֶׁם
It will be that whoever of the families of the Earth does not up to Jerusalem to worship the King, the L-rd of Hosts, will have no rain.
This continues from the preceding verse, talking about the holiday of Succos. The second mishna in Rosh Hashana (1:2) tells us that on the world’s water is judged on Succos. (A full discussion of the matter ensues in the Talmud in Rosh Hashana 16a.) One of the sources for this information is our verse in Zechariah, which details the fate of nations who, in the future, will not come to honor G-d on the holiday of Succos.