In reply to your 6/15 post Ascension Day, Feast of Weeks -Another Look In Relation to the Count to Acts Pentecost Along With Some Dates and in regards to your statement below that "we don't know for sure" Jesus ascended into heaven 40 days after his resurrection, please reread your statement below & then the Bible verses from Acts 1:9-11 which indicate that we can be sure that Jesus ascended into heaven 40 days after his resurrection which means the Feast of Weeks that happens on the 50th day is not the same day of Jesus' ascension day on the 40th day. So Jesus did see his disciples on the 40th day which was his ascension day.

YOUR STATEMENT:

"Luke wrote that "they" saw Him during forty days/over forty days.  We know that according to Luke 24:36-43 and John 20:19-23 Jesus appeared to His disciples on Resurrection Sunday evening in Jerusalem.  Many believe the forty day count started then.  It has been assumed He ascended 40 days after His resurrection.  We don't know that for sure."

 

"I've wondered if there wasn't a gap after the 40 days.  And that Jesus saw them the last time on Ascension Day.  We know, according to the Gospels, that Jesus appeared to them on at least ten different days."

BIBLE VERSES:

Acts 1:3-5,"To whom also he shewed himself alive after his passion by many infallible proofs, being seen of them forty days, and speaking of the things pertaining to the kingdomof God: And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me. For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence."  

For the disciples to wait in Jerusalem from Jesus' ascension day on the 40th day until the New Wine Festival "is not many days hence" because it was 60 days away. However, for them to wait in Jerusalem for the Feast of Weeks (Pentecost) was only 10 days away. 

Then after Jesus had spoken all the things recorded in Acts 1:6-8, according to Acts 1:9-11 he was then taken up to heaven. "And when he had spoken these things, while they beheld, he was taken up; and a cloud received him out of their sight. And while they looked stedfastly toward heaven as he went up, behold, two men stood by them in white apparel; Which also said, Ye men of Galilee, why stand ye gazing up into heaven? this same Jesus, which is taken up from you into heaven, shall so come in like manner as ye have seen him go into heaven." This was Jesus' 2nd ascension into heaven.

Jesus was resurrected on the third day which was on the Feast of First Fruits of the wave offering of the barley sheaf when he ascended into heaven the 1st time on that day because according to John 20:17, he told Mary Magdalene, "Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God." Then 8 days later according to John 20:26-27, he said to doubting Thomas, "Then saith he to Thomas, Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing." 

All the male Jews were required to go up to Jerusalem 3 times a year with the Feast of Weeks being one of them.  It was not a requirement for them to go to the New Wine Festival which means those living in faraway countries would have to come back to Jerusalem 60 days after Jesus' Ascension Day or 50 days after the Feast of Weeks if Pentecost really was the New Wine Festival.  

There were men dwelling in Jerusalem out of every nation under heaven that heard the disciples speak in tongues as stated inActs 2:5 which says, "And there were dwelling at Jerusalem Jews, devout men, out of every nation under heaven."  However, according to Acts 2:9, there were others there that were  "dwellers in Mesopotamia, and in Judaea, and Cappadocia, in Pontus, and Asia, Phrygia, and Pamphylia, in Egypt, and in the parts of Libya about Cyrene, and strangers of Rome, Jews and proselytes, Cretes and Arabians, we do hear them speak in our tongues the wonderful works . So it just wasn't the Jews dwelling in Jerusalem in Judaea that heard the disciples speak in other tongues, but those Jews dwelling in Judaea outside of Jerusalem along with all the Jews who were dwellers in the faraway countries that came to Jerusalem to celebrate the Feast of Weeks as required by the Lord to go up to Jerusalem at the Feast of Weeks.

Why did Jesus come back to Earth and stay here for another 40 days before ascending into heaven a 2nd time?  I think it had to do with having everyone believe that he was resurrected from the dead and alive.  If he had ascended into heaven the 1st time and stayed there, they may have thought he was dead and someone stole his body.  Also if you recall the Israelites spent 40 years in the wilderness for their unbelief because after the spies returned after spending 40 days in Canaan, the people were afraid of the giants and fell into unbelief because they did not think that the Lord was capable of bringing them safely into Canaan even after all the miracles that He performed.  So after all the miracles Jesus performed and people witnessed, his resurrection from the dead on the 3rd day was the greatest miracle that God the Father performed.