Mortality is probably the major theme in this poem. It's all about the speaker's attitude toward her death and what the actual day of her death was like. Dickinson paints a picture of the day that doesn't seem too far from the ordinary (that is, if you're used to having a guy named Death take you out on dates). The speaker isn't scared of death at all, and seems to accept it.
The speaker isn't really relaxed about her experience with Death; she's terrified.