This refers to Jesus' second coming, his return to earth from heaven.
# what is that to you?
This remark appears in the form of a question to express a mild rebuke. AT: "that is not your concern." or "you should not be concerned about that." (See: [[rc://en/ta/man/translate/figs-rquestion]])
# among the brothers
Here "the brothers" refers to all the followers of Jesus.