"The Ghost Rider"

In "The Ghost Rider", 13 October 1957, the sheriff tells Bret Maverick that Mary Shane has been dead for 9 days. The next day Bret Maverick rides out to where he last saw her, finds his coat, and sees her tombstone saying that Mary Shane died on June (3, I think) 1872.

So that means that Bret Maverick met Mary's ghost on June 12, 1872, and the next day was June 13, 1872.

The town is White Rock, and since Mary talked about traveling from Montana to Texas, it is probably in Texas.