The 15th day of Aviv starts Chag HaMatzot (Festival of Unleavened Bread) referenced in [Leviticus 23:6-7] which begins בָּעֶ֔רֶב Ba-Erev “In-Evening” (sunset) of the 14th of Aviv - NOT the sunrise of the 14th day. [Exodus 12:10] occurs during Boqer בֹּקֶר (sunrise) of Pesach - still the 14th Day of Aviv.
the "Pass [over]" plague only occurred בָּעֶ֔רֶב Ba-Erev (in-[the] evening) at the start of the 14th Day of Aviv, the Israelites fled Mitzrayim at night לַ֗יְלָה early on the 14th Day of Aviv [Exodus 12:31].
At sunrise on Pesach (when the remains of the Paschal Lamb are burned) the day is still the 14th. - as stated in [Exodus 12:10] (וְלֹֽא־תוֹתִ֥ירוּ מִמֶּ֖נּוּ עַד־בֹּ֑קֶר וְהַנֹּתָ֥ר מִמֶּ֛נּוּ עַד־בֹּ֖קֶר בָּאֵ֥שׁ תִּשְׂרֹֽפוּ) - Boqer בֹּקֶר is a SUNRISE - not a sunset.
Exodus 12:18 states
"In the first [month], on the fourteenth day of the month in the evening, you shall eat unleavened cakes, until the twenty first day of the month in the evening." ( בָּֽרִאשֹׁ֡ן בְּאַרְבָּעָה֩ עָשָׂ֨ר י֤וֹם לַחֹ֨דֶשׁ֙ בָּעֶ֔רֶב תֹּֽאכְל֖וּ מַצֹּ֑ת עַ֠ד י֣וֹם הָֽאֶחָ֧ד וְעֶשְׂרִ֛ים לַחֹ֖דֶשׁ בָּעָֽרֶב )
Full Moon appeared "in the evening" of 14th / Start of 15th Day of Aviv - starting the celebration of Chag HaMatzot.
"The people picked up their dough when it was not yet leavened, their leftovers bound in their garments on their shoulders." ( וַיִּשָּׂ֥א הָעָ֛ם אֶת־בְּצֵק֖וֹ טֶ֣רֶם יֶחְמָ֑ץ מִשְׁאֲרֹתָ֛ם צְרֻרֹ֥ת בְּשִׂמְלֹתָ֖ם עַל־שִׁכְמָֽם )
"They baked the dough that they had taken out of Egypt as unleavened cakes, for it had not leavened, for they were driven out of Egypt, and they could not tarry, and also, they had not made provisions for themselves." ( וַיֹּאפ֨וּ אֶת־הַבָּצֵ֜ק אֲשֶׁ֨ר הוֹצִ֧יאוּ מִמִּצְרַ֛יִם עֻגֹ֥ת מַצּ֖וֹת כִּ֣י לֹ֣א חָמֵ֑ץ כִּי־גֹֽרְשׁ֣וּ מִמִּצְרַ֗יִם וְלֹ֤א יָֽכְלוּ֙ לְהִתְמַהְמֵ֔הַּ וְגַם־צֵדָ֖ה לֹֽא־עָשׂ֥וּ לָהֶֽם )
- Evening of 14th / Start of 15th | day #1.
- Evening of 15th / Start of 16th | day #2.
- Evening of 16th / Start of 17th | day #3.
- Evening of 17th / Start of 18th | day #4.
- Evening of 18th / Start of 19th | day #5.
- Evening of 19th / Start of 20th | day #6.
- Evening of 20th / Start of 21st | day #7.
- Evening of 21st of Aviv ended the 7-day festival of Chag HaMatzot.