If Spike had known, he would've been just mildly disappointed at the missed opportunity for hilarious mocking. Skipping the vampire costume was probably the right call, in the end, for many reasons. Spike isn't dressed up either — this is his day off — but that doesn't mean he can't take in the atmosphere over a walk and a warm cup of blood.
Spotting Jack, he tenses. Their last meeting ended weirdly and abruptly and he's not really looking forward to the discussion that'll inevitably follow.