Don't know much about that specific date, but, AFAIK, "currently not on sale" just means that tickets are still available, but are not allocated for sale yet; when they get sold out, they will be marked as "sold out".
In detail: tickets often are not offered for sale all at the same time, but are sold in smaller batches, the next batch only coming out after the current one is fully sold.
So, "currently not on sale" means that the current batch was sold, and they are waiting for the next batch to be allocated soon - in a few days, depending on the sales strategy they chose.
Best to keep checking the site a couple of times a day, the tickets should reappear.
(... any way the wind blows ...)