Including the title, the word death appears in "When Death Comes" five times! It's referenced directly, though not by name, several more times. If you wanted to sum up the basis of the poem to a friend you could probably just call it a meditation arising from the thought of death. Of course, death only means something in combination with life, which could also be called our speaker's main concern. Much of the work of this poem seems to be challenging the idea of death as something to be feared, seeing death instead as a way of defining how we live, and as something to be approached with the same sense of wonder and possibility that makes us appreciate life.
The poem opens with the repeated phrase "when death comes" and ends (the second and third to last stanzas) with the repeated phrase "when it's over," with "it" referring to life. This transition demonstrates the arc of the poem, which transforms a contemplation of death into a contemplation of life.
In order for the speaker to consider approaching death without fear, she needs the possibility of eternity. Thus, the only way she can handle death so calmly is by transforming it into something else, a cottage of darkness where life can, in some way, continue. She does not overcome her fear, but rather convinces herself that death is not in fact death.