By reading the poem, I would conclude that there has been a loss in the family. There is no mother in the story, so perhaps she has died. The man in the picture the child draws has buttons that look like tears. However, it is also possible that the man has died or left, and perhaps the child simply does not know. The story is set in September, a time of change, which may be a central theme in the poem.
Repeated references to tears throughout the poem stress the deep sadness present, as do the tears the grandmother tries to hide with laughing and talking, as if to distract the child and perhaps cope herself. "Equinoctial" tears refer to the equinox (longest and shortest days of the year), which in light of the deep sadness I would assume refer to the longest day because the pain is so great and seems to go on and on.
The rain also supports the sense of deep sadness.
There is reason to believe that the almanac is being presented as a fortunetelling "device." The grandmother believes the almanac has foretold her tears of sadness the same way it predicted the rain that falls on the roof, but only she can know this; perhaps this speaks to the fact that she is of another generation that sees magical or portentous signs, things that the younger generations might not pay attention to.
The allusion to magic may be further described in that the almanac hovers like a bird above the heads of the grandmother and child (even though on a string), half opened. The hovering almost seems magical.
The child's stove speaks to the almanac indicating that this tragedy (whatever it is) was foretold ("It was to be..."), and the almanac states that what is within its pages cannot be changed ("I know what I know").
The house drawn is rigid, but the path from the house winds; we get the sense of the house that is angular and fixed, but the path from the house changes directions as it winds. This is where the man is placed in the picture.
As the book hangs open, little moons fall from its pages into the flower bed the child has drawn in her picture. Though the flowers in the front of the house would generally symbolize something gentle or beautiful, this poem has little of those kinds of images. However, the rain on the flowers (even tears shed) will promote growth, while the fall will bring about a change in them.
The falling moons, which are images that are found within an almanac in all its lunar phases, may symbolize change that is taking place in the lives of the grandmother and child, as the moon often symbolizes change.
The grandmother knows about the changes, but the child does not.
The second picture the child draws gives no clue to what lies ahead, but remains mysterious. It cannot foretell the future—though it would seem that the almanac can.