What will be closed for the May bank holiday

I presume you mean you will arrive in London. If so, maybe you could google the sites or museums of interest to you . I just googled the National Art Gallery and it does not close on the May bank holiday, for instance. Will you be arriving on a long haul flight? If so, you may not want to be visiting too many places, anyway.

Most everything you'll want to see will be open but expect longer lines. http://www.timeout.com/london/things-to-do/bank-holiday-in-london Banks are closed but few businesses. In fact many have sales that day. Don't fret over this. Great city!

Thank you, I was worried I might wind up in a ghost town on my first day in London

Happy travels!

London is never a ghost town. Even on Christmas Day. In fact because as it's a bank holiday expect a bit more congestion.
Arrive, unpack, explore!