usage of "come fall"

I saw that "come fall" is used in the following sentences:
It will be easier to wear runway clothing to the office come fall.
Come fall, CBS will do what CBS does best: play it safe.
Still, she added, no one knows the course the virus will take come fall, the traditional flu season.

I understand what these sentences mean, but what is the usage of "come fall"? Is the word "come" here is a verb or an adjective?