Between the Lord's resurrection and ascension He wanted to convey to His Disciples His New Covenant Kingdom teachings.

However, He knew that in order to receive His teachings the Disciples would need the help of the Holy Spirit.

But, as  John 7'39 says, the Holy Spirit "was not yet given, because Jesus was not yet glorified".

So Jesus did something unique. "He breathed on them and said to them, 'Receive the Holy Spirit'" (John 20'22).

Luke confirms this at the beginning of the Book of Acts:

"... until the day in which He was taken up, after He through the Holy Spirit had given commandments to the apostles whom He had chosen,

to whom He also presented Himself alive after His suffering by many infallible proofs, being seen by them during forty days and speaking of the things pertaining to the kingdom of God," Acts 1'1-3.

The Lord departed, leaving them time to be together and prepare their hearts to receive.