You can catch the livestreamed keynote here, on the Channel 9 home page, from 4pm BST tomorrow, i.e. 10 May 2017.
For anyone interested in the full range of BUILD sessions, they're listed in the catalog here. 287 different talks or forums show the massive breath of this developer conference, though I can't emphasise the 'developer' aspect enough - some sessions will be livestreamed or available on demand on Channel 9, but you have to be a hardcore developer to really profit from any of them. Even the keynote won't be trivial to follow!