At the end of Chapter 27, Scout says about the Halloween pageant at school, "Jem said he would take me...thus began our longest journey together". These lines alone communicate a clear sense of foreboding, focusing the sense of ominousness the author has carefully crafted throughout the Chapter.
Bob Ewell has won the case stemming from the unjust accusations he has levied against Tom Robinson, but he has not won the respect of the town. Although their ingrained prejudice against blacks resulted in Robinson's conviction, the people still recognize Bob Ewell for what he is, lazy, degenerate, and a fool. Shunned as much as ever, Bob Ewell acts out his dissatisfaction in sinister ways, blaming Atticus for the loss of his job with the WPA, attempting to break into Judge Taylor's house, and harrassing Tom Robinson's widow as she walks to work on the main road. Ewell is a dangerous, angry man, and there is no telling what he may do next.
Aunt Alexandra feels this sense of imminent danger on Halloween night, when she is talking to Scout and stops speaking in mid-sentence. When Scout asks what is wrong, she says, "oh, nothing, nothing...somebody just walked over my grave". She then "put(s) away from her whatever it was that gave her a pinprick of apprehension", and Scout leaves for the pageant with Jem on "their longest journey together" (Chapter 27).
also on top of what the other answer said its halloween night which usually means monsters and ghosts and evil things in general which helps create the ominous atmosphere.