Similar to this deal in 2015, you can purchase $20 tickets to certain performances with a few caveats, the main one being that tickets only go on sale 2 weeks before the season starts (a lot of tickets will be sold by this point). Someone mentioned in the last deal that they cannot pick out which seats they had but I can't confirm that. Max 2 tickets per person.
There are 4 shows in the year but the first one ended on April 9:
If you and your friends buy tickets at the same time, you will most likely be seated together.
"Once in Royal David's City" has finished.
It is worth subscribing to the mail list too.