Exodus 12:18

עד יום האחד ועשרים UNTIL THE TWENTY-FIRST DAY — Why is this stated? Has it not already been stated, (v. 15) “Seven days [shall ye eat unleavened bread]” (beginning on the fifteenth day, as Rashi shows in his comment on v. 14 and therefore terminating on the twenty-first)? The answer is: Since it is said in that verse, “[seven] days [shalt thou eat unleavened bread]”, we may ask whence do we derive that the prohibition of eating leavened bread extends also to the nights of these seven days? Scripture therefore states here, “[Ye shall eat unleavened bread] until the twenty-first-day in the evening” (Mekhilta).

In the first month, on the fourteenth day of the month at evening, shall you eat unleavened bread, until the twenty first day of the month at evening.

בָּרִאשֹׁ֡ן בְּאַרְבָּעָה֩ עָשָׂ֨ר י֤וֹם לַחֹ֙דֶשׁ֙ בָּעֶ֔רֶב תֹּאכְל֖וּ מַצֹּ֑ת עַ֠ד י֣וֹם הָאֶחָ֧ד וְעֶשְׂרִ֛ים לַחֹ֖דֶשׁ בָּעָֽרֶב׃