The Japanese Emperor will abdicate the throne At May 1st 2019.
Therefore, three changes will occur.
[1st]
Change [Emperors Birthday(天皇誕生日)] to Feb 23th.So,
since 1949 to 1988 : Apr 29th,
since 1989 to 2018 : Dec 23th,
since 2020 : Feb 23th.(The Emperor will abdicate at May 1st 2019. So The Emperor Birthday is not in 2019)
[2nd]
Add [Coronation Day(即位の日)]. at May 1st only 2019.
[3rd]
Add [Enthronement Proclamation Ceremony(即位礼正殿の儀)]. at Oct 22th only 2019.
The Japanese Emperor will abdicate the throne At May 1st 2019. Therefore, three changes will occur.
[1st] Change [Emperors Birthday(天皇誕生日)] to Feb 23th.So, since 1949 to 1988 : Apr 29th, since 1989 to 2018 : Dec 23th, since 2020 : Feb 23th.(The Emperor will abdicate at May 1st 2019. So The Emperor Birthday is not in 2019)
[2nd] Add [Coronation Day(即位の日)]. at May 1st only 2019.
[3rd] Add [Enthronement Proclamation Ceremony(即位礼正殿の儀)]. at Oct 22th only 2019.
https://www8.cao.go.jp/chosei/shukujitsu/gaiyou.html