Ezekiel 40:1

בְּעֶשְׂרִים וְחָמֵשׁ שָׁנָה לְגָלוּתֵנוּ בְּרֹאשׁ הַשָּׁנָה בֶּעָשׂוֹר לַחֹדֶשׁ בְּאַרְבַּע עֶשְׂרֵה שָׁנָה אַחַר אֲשֶׁר הֻכְּתָה הָעִיר בְּעֶצֶם הַיּוֹם הַזֶּה הָיְתָה עָלַי יַד ה’ וַיָּבֵא אֹתִי שָׁמָּה

In the twenty-fifth year of our exile, on the New Year, on the tenth day of the month in the fourteenth year after that the city was struck, on that day, the hand of G-d was upon me, and He brought me there.

The tenth day of the month? But everyone knows that Rosh Hashana is on the first day of the month! Actually, the Talmud in Arachin (12a) explains that this was during Yoveil – the Jubilee year. Yoveil has special rules, like land reverts to its original owners and slaves go free. The special rules of Yoveil do not take effect until Yom Kippur, which is the tenth day of the month. So Yom Kippur is really the first day of the Jubilee year.