וַיֹּ֨אמֶר יְהוֹנָתָ֜ן אֶל־דָּוִ֗ד יְהוָ֞ה אֱלֹהֵ֤י יִשְׂרָאֵל֙ כִּֽי־אֶחְקֹ֣ר אֶת־אָבִ֗י כָּעֵ֤ת ׀ מָחָר֙ הַשְּׁלִשִׁ֔ית וְהִנֵּה־ט֖וֹב אֶל־דָּוִ֑ד וְלֹֽא־אָז֙ אֶשְׁלַ֣ח אֵלֶ֔יךָ וְגָלִ֖יתִי אֶת־אָזְנֶֽךָ׃
Rashi on I Samuel
On the third day. Of the following day. That means the third day at this time, for tomorrow is the new moon, and tomorrow my father might not wonder; but he will wonder on the third day.