The word 'selfsame' refers to a specific commemorative date. The selfsame day is a signal that God is faithfully in control of time over multiple centuries.
The wavesheaf offering is reckoned from the weekly Sabbath within the Days of Unleavened Bread. It had specific requirements that were not met in Joshua 5.
The 'very same' or 'selfsame' day is a memorial of a past event, typically on the same date, including several of God's appointed times and pronouncements.
The term 'selfsame day' refers to the covenant God made with Abraham 430 years before the Exodus, which occurred on the day after the Passover.
The same God who exercised vigilance and care over our forebears does so over us. Israel's leaving Egypt signifies maintaining the relationship with God.
The timing of Christ's crucifixion does not coincide with the Passover, but instead lines up with the covenant God made with Abraham, marking a major fulfillment.
How does one count to Pentecost when Passover is on a weekly Sabbath, making the Last Day of Unleavened Bread the only other available Sabbath to begin the count?
Because Pentecost does not have a specific date, God commands us to count from the day after the weekly Sabbath falling within the Days of Unleavened Bread.
The Night Much to be Observed is a memorial of the covenant with Abraham, and God's watchfulness in delivering ancient Israel as well as spiritual Israel.
We have to learn to rely on God to get us out of strait and difficult situations, realizing that God may want to help us to develop a backbone and mature.