Actually, the musical will be a multiple day event. Here is the complete schedule, as copied from the MySpace blog:
"ONE WEEK ONLY! AN INTERNET MINISERIES EVENT!
Dr. Horrible's Sing-Along Blog will be streamed, LIVE (that part's not true), FREE (sadly, that part is) right on Drhorrible.com, in mid-July. Specifically:
ACT ONE (Wheee!) will go up Tuesday July 15th.
ACT TWO (OMG!) will go up Thursday July 17th.
ACT THREE (Denouement!) will go up Saturday July 19th.
All acts will stay up until midnight Sunday July 20th. Then they will vanish into the night, like a phantom (but not THE Phantom - that's still playing. Like, everywhere.)"
Source: Dr. Horrible on MySpace