Yesod shebiTeferet יסוד שבתפארת

The mystical significance of the 20th of the Omer in the teachings of the Vilna Gaon from Ari Kahn

An audio explanation is available here:

http://www.yutorah.org/lectures/lecture.cfm/735258/Rabbi_Ari_Kahn/Yom_Yerushalayim_-The_Vilna_Gaon-s_mystical_doctrine_-_the_beginning_of_the_end_of_days-

or here:

https://itunes.apple.com/il/podcast/rabbi-ari-kahn-holiday-shiurim/id612367256?mt=2
under "yom yerushalayim"

or here

http://shiur.rabbiarikahn.com/Shiurim/Holiday/Iyar/Yom_Yerushalayim.mp3