When Cassie meets the missionary on the cemetery, the landscape is covered in deep snow. A little later - arguably on the same day - when she and Cole arrive at the rendezvous point the missionary has invited her to, the snow has disappeared entirely, and the ground is fairly dry. Even if a sudden heatwave had occurred and melted most of the snow within a few hours, there would still be traces of it left.
On the missionary's invitation it states that the gathering of the Faithful is set for December 22, at 8 pm. When Cole, Cassie and Agent Gale arrive at the meeting point, the evening sun is still shining. This would not be unusual during the summer months. However, on one of the shortest days of the year in the north of the US, at 8 pm, the sun would have set several hours earlier.