Mystic Code: Memories of the Lunar Sea

https://grandorder.gamepress.gg/quest/mystic-code-quest-memories-lunar-sea
"Available for a limited time during the Fate/EXTELLA Release Commemoration Campaign 07/20~7/27/2017."
https://fategrandorder.fandom.com/wiki/Memoria_of_Lunar_Mare
It was added to JP's RP shop back in 2018 so we can expect it there in 2020 :/
Let's hope that DW will be merciful and re-release it with small campaign that celebrates release of English version of /Extella Link
Butwe can only hope since JP didn't get it with their Extella Link campaign

As far as the Japan schedule goes...
http://fate-go.cirnopedia.org/news.php?id=2018-045#nav
Last year the Fate/Extella Link Release Commemoration Campaign was held in which it was possible to do the quest for this mystic code again.
Fate/Extella Link will be released on March, 19th in the west. So maybe we'll get lucky and get it by then.
But I can only make a guess.
I'm hoping for it as well, by the way. I really need a good buff for my arts Servants, since I don't have Tamamo.