Due to the epic nature of the story we’ve been working on, Harry Potter & the #CursedChild will be in two parts! @HPPlayLDN
— J.K. Rowling (@jk_rowling) September 24, 2015Here's an explanation of the breakdown, from the play's producers Sonia Friedman & Colin Callendar. They're still working out logistics on the actual schedule since it's now two 2-hour long plays.
Friedman: "It can be seen on consecutive days in the same week or in some instances on the same day... [the story is] too long to be told in a traditional length and it became inevitable that it had to be in two parts."
Callender: ‘Access is important to us and certainly we want audiences to see the plays in the same week.’
Tickets go on sale in late October, and the play opens in London next summer (and yes, that's just the debut - producers already said it'll tour worldwide, especially Broadway, in coming years). Yes, this shit is gonna cost a fortune.
Sources: https://twitter.com/jk_rowling/status/647153651681751041
https://twitter.com/BazBam/status/647185850921566209
https://www.pottermore.com/news/cursed-child-announcement
https://twitter.com/HPPlayLDN/status/647154353275584512