פירוש על עבודה זרה 73:12

The rabbis and R. Judah Nesi’a disagree if the husband says, “If the sun comes out.” R. Judah Nesia’ says that in this case the date proves that his intent is for the divorce to go into effect retroactively once the sun comes out. It is like saying, “From today if I die.” So even if he dies at night, she is divorced. The other rabbis hold that that this is like saying “when the sun comes out.” So if he dies at night, the divorce never went into effect.
People may be wondering what all this hullabaloo is about—after all, she is either a widow or a divorcee. The issue is levirate marriage. If she is a widow, she might have to marry her dead husband’s brother. If she does not want to, she could be left without the ability to remarry.