Severus found himself oddly nervous as he Side-Alonged Al and himself to the Apparation coordinates in Lily's note. It was different from the way he'd felt before the dinner with Harry and Ginny; he'd had a very good idea of what to expect: he knew that Ginny's presence boded no good.
He honestly wasn't sure what to make of Lily's invitation. If she'd invited Al and himself to the Leaky Cauldron or somewhere else where James wouldn't be joining them, he would've felt only anticipation. But her note sounded as though James was going to be there, and frankly Snape couldn't imagine James wanting to be anywhere near him - unless he intended to use homeground advantage to try to hex him, but if that was his plan he'd never want Al there.
Really, it made no sense at all. And situations he couldn't parse were much more disconcerting, than situations where he knew which species of shit was about to hit the fan, and could plan how to duck the worst splatters.
With an irritable shake of his head at his own wandering thoughts, he stepped up to the front door of the surprisingly spacious house, and knocked.