וַיֹּֽאמֶר־ל֥וֹ יְהוֹנָתָ֖ן מָחָ֣ר חֹ֑דֶשׁ וְנִפְקַ֕דְתָּ כִּ֥י יִפָּקֵ֖ד מוֹשָׁבֶֽךָ׃
And Jonathan said unto him: ‘To-morrow is the new moon; and thou wilt be missed, thy seat will be empty.
Read midrashic interpretation of I Samuel 20:18: and classical rabbinic exegesis.