In “Out, Out—” by Robert Frost, what is ironic about the time of day the accident occurs?

The irony here is that the accident happens at the end of the work day, at a time when the boy might have been dismissed to take an extra half hour to himself. In fact, his sister is even there saying that supper is ready. Because he is not allowed to leave, however, he is still working later, when he is injured. It may even be the fatigue of a long day that led to the accident. If the boy had only been able to go play, he would still be alive, with an intact hand. Such is the irony of the poem, with the speaker's rueful wish that they might have told him to "call it a day." The speaker sympathizes with his character, portraying the events of the poem as out of the boy's control.

The tragedy of the poem is born of the fact that the accident occurs so close to the end of the workday. It happens at sunset (line 6), when the boy's sister is interrupting his work, saying supper is ready (line 14).
The poet's method throughout the text hinges upon dramatic irony. Indeed, the speaker's only direct incursion occurs in line 10 (via the first person pronoun "I"), where he laments how preventable the accident was: "Call it a day, I wish they might have said." If only the boy had been allowed to be a boy, and not just a worker, and if working time had been distributed more humanely, then the boy would not have lost his hand but made the most of his childhood in a rare, precious half hour of free time (line 11).