Who was Peter preaching to on the day of Pentecost?

On the day of Pentecost, Peter was preaching to Jews. He said Christ, being delivered by the determinate counsel and foreknowledge of God, was taken and by wicked hands crucified and slain. This is recorded in Acts chapter 2, verse 23.

Les explains that Peter was preaching to Jews on the day of Pentecost, as recorded in Acts 2:23. Peter declared that Christ was delivered by the determinate counsel and foreknowledge of God, and that they had taken Him and by wicked hands crucified and slain Him. Les connects this to eternity past, before anything was called into being, when the triune God held a counsel. Though they did not need to deliberate, the three persons of the Trinity agreed to create the universe and set everything in motion, placing one little planet like a speck in the midst of it.

Acts 2:23

Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain:

Did Peter know about the reserved place and adoption at Pentecost?

No. Peter did not know about the reserved place and adoption. Peter could have said God had reserved a place and had adoption waiting, but Peter did not know that; God did. Those doctrines of grace remained unknown until God revealed them to Paul.