What: Solar Eclipse Viewing Party! Will occur on the lawn, dotted with tents for shade, and light refreshments (lemonade, wine, sandwiches, cakes) will be provided, with lawn games and general chatter for additional occupation. The house will be kept off-limits to visitors, though that doesn't always stop folk.
Who: All upper-class families (save, conspicuously, for those led by a mudblood) would have received invitations, as would the pureblooded middle-class families. Their children (6th-7th year and pre-Hogwarts) would also be invited, should their parents be willing to make the necessary arrangements with the school. Hogwarts staff would likely be absent, as they would be teaching; Pendergast, Macnair, and Lestrange children of any age would be present.
Where: Technically elsewhere, at a relatively unused Lestrange country manor. A floo powder system will be set up through the Lestrange's fireplace in Wellingtonshire that connects to their country estate.
Why: Because I realized that Olivia is supposed to be all social and throw events and the like and she...hasn't done either. At all. Ever. (I suck.)
When: April 22, 1882