These adult birds are easy to identify with their white belly, two white outer tail feathers and pink bill. The male is more gray than the female who has a brown tint to her feathers.
The brown coloring suggests that this may be a female or a more mature juvenile. It is difficult to see the markings under the beak to make the final determination.

This little bird on the other hand is clearly a juvenile. Many people mistake the juvenile for a completely different bird because it has so many colors running through its feathers. The mottled colors on the upper breast are surely those of a juvenile.
Winter, Spring, Summer or Fall this bird is everywhere. However, it should not be taken for granted because it is very entertaining to watch and where there are juncos there are often other small species that join their group.