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.
That's right, two opposite themes – Mortality and Immortality – occupy this poem. We find out that the memory of the speaker's death day is being told centuries into the afterlife. So, in this poem, Dickinson explores the idea of perpetual life. In this poem there is life after death, which offers an explanation as to why the speaker's so calm about everything. Death's not the end, just one step closer to eternity.
Even though Dickinson doesn't specifically name it, the speaker is in Heaven.
The horses mentioned in the poem were actually angels, carrying the speaker to the afterlife.
Well, the speaker is a ghost, which means Dickinson had to believe in some sort of life after death (and we do know that she grew up in a Christian family). But she leaves specific religious references out of the poem, and we don't know if the speaker is recalling the memory of her death from Heaven, Hell, or somewhere else; we only know that it's a place beyond this world.
The formality of their slow progressions is supposed to mirror a traditional religious death procession.
The spirituality of the speaker belongs to no formally-established religion, but is her own personal belief system.
The poem doesn't really address love head-on, but it certainly gives us a glimpse into courtship (a.k.a. dating) and romantic love. If you exchange "Tom" or "Joe" for "Death" here, this could be a pretty good example of dating for the 1800s. The speaker's tone in the poem makes the reader believe the speaker does not fear death, but feels the opposite toward it. If the poem did not explore death with an underlying theme of love, the acceptance of death might eventually be hard for the reader to believe.
The speaker knew about the date beforehand and that's why she's dressed up and not at all surprised to see Death.
The speaker hasn't really passed into an afterlife, but lives in the "house" with Death.