As soon as we see the word "Widow" in the title, we know that death has knocked in this speaker's door. Beginning with this announcement of the husband's death, and ending with an imagined or longed for death for our speaker, "The Widow's Lament in Springtime" is bookended by the light at the end of the tunnel. In between, well, it's still pretty much all about that, too. Even the renewal of spring has death built into it: in order to have spring, there must be a fall and winter, when death, rather than birth, is the main event.
By relating death to the cycle of seasons and by imagining our speaker's passing as a union with the natural world, the poem suggests that death itself may not be an end but rather a path to rebirth.
Our speaker isn't sad that her husband died, merely that he's not around anymore. She has a death wish at the end of the poem because she's hoping to reunite with her husband in some sort of afterlife.