Thoughts on the following

From 1 Cor 5:7 for also our passover for us was sacrificed -- Christ,
Lev 23:5 in the first month, on the fourteenth of the month, between the evenings, is the passover to Jehovah;

Notice the fourteenth is not said to be a holy convocation.
It is the death of Christ the Passover Lamb, the death of the Red Heifer without the camp.

and on the fifteenth day of this month is the feast of unleavened things to Jehovah; seven days unleavened things ye do eat; on the first day [15th] ye have a holy convocation, ye do no servile work; and ye have brought near a fire-offering to Jehovah seven days; in the seventh day [21st] is a holy convocation; ye do no servile work.'

Because of the death of the Passover Lamb, the Red Heifer
From 1 Cor 5:7 according as ye are unleavened, for also our passover for us was sacrificed -- Christ,

1 Cor 5:8 so that we may keep the feast, [With a holy convocation on the 15th and a holy convocation on the 21st] not with old leaven, nor with the leaven of evil and wickedness, but with unleavened food of sincerity and truth.

I posted this for the following questions.

Why did the feast of unleavened consist of two days of holy convocation and seven total days?
Did it have anything to do with cleansing of the water of separation on the third day and the seventh day? See Numbers 19:11,12

Christ died and was raised from the dead on the third day, made alive 1 Cor 15:20-22 becoming the first-fruit of them that slept and Lev 23:11 then he hath waved the sheaf before Jehovah for your acceptance; on the morrow of the sabbath [the third day] doth the priest wave it. John 20:17 Jesus saith to her, 'Be not touching me, for I have not yet ascended unto my Father; and be going on to my brethren, and say to them, I ascend unto my Father, and your Father, and to my God, and to your God.' [the third day accepted for you.]

Then fifty days from that morrow of the sabbath the Holy Spirit, the promise of our eternal inheritance was given, the feast of first-fruit of the Spirit, a holy convocation.

Heb 9:13-15 for if the blood of bulls, and goats, and ashes of an heifer, sprinkling those defiled, doth sanctify to the purifying of the flesh, how much more shall the blood of the Christ (who through the age-during Spirit did offer himself unblemished to God) purify your conscience from dead works to serve the living God? And because of this, of a new covenant he is mediator, that, death having come, for redemption of the transgressions under the first covenant, those called may receive the promise of the age-during inheritance,

Also

1 Peter 3:21 also to which an antitype doth now save us -- baptism, (not a putting away of the filth of flesh, but the question of a good conscience in regard to God,) through the rising again of Jesus Christ, [on the third day]