To stress the first syllable would be to add emphasis that the event is imminent, now, tonight - not tomorrow. I would personally just add emphasis to the second syllable, and say, "it's toNIGHT, you know!" but in another example, it might be more common to say, 'No, it's TOnight, not tomorrow!" Of course, I prefer my own emphatic suggestion of toNIGHT, but others may find the TOnight more to their ear. They are wrong of course, and should be shunned.
(I'm kidding, you realize?)