In the reading this week, John 15:26 makes it clear that the apostles didn’t have full access to the gift of the Holy Ghost. The Bible Dictionary entry on “Holy Ghost” says, “For some reason not fully explained in the scriptures, the Holy Ghost did not operate in the fulness among the Jews during the years of Jesus’ mortal sojourn (John 7:39; 16:7).”

So how long was it before they received the gift of the Holy Ghost? The apostles received this gift on the day of Pentecost, likely about one week after Jesus ascended into heaven (remember He spent 40 days with His disciples after His resurrection– see Acts 1:3). 

Pentecost was an already-existent holiday celebrated 50 days after Passover. So Jesus was among His disciples until about a week before Pentecost, which is the day the Holy Ghost was given to the disciples. (See Acts 2:1-3)