This is a very unique, homey annual transmission aimed at British researchers in Antartica on their "shortest" day of the year. Well wishes are sent by friends, family, and co-workers. I think "Happy Birthday" was sung and "Here Comes the Sun" was played last year. The best I could glean from online information (did not hear back from a BBC Inquiry) is that they will broadcast it at 2130z for 30 minutes on 21 June 2017. The frequencies will be 5985, 6035, 7230, and 7360. I had success with 7360 last year (posted here about it, you can search), 7230 appears to be a new frequency for the special event.