Day emptiness is the same as ten-day cycle void (旬空)
From the classics
旺不为空，动不为空，有日建动爻生扶者亦不为空，动而化空、伏而旺相皆不为空；月破为空，有气不动亦为空，伏而被克亦为空，真空为空。
What it means
Day emptiness is the same as ten-day cycle void (emptiness of a ten-day cycle). According to the ten-day cycle (ten-day cycle) containing the divination day, the two Earthly Branches missing from that cycle are considered empty. For example, in the Jiazi cycle, 戌 and 亥 are empty. If the useful spirit falls in emptiness, it indicates that the matter is empty and unreal, and plans are difficult to accomplish. However, a strong line is not considered empty; a moving line is not considered empty. When the line leaves the cycle or meets a day that fills it (by clash or by value), that is the time of fulfillment. If a harmful spirit falls in emptiness, its harm can be reduced; if an auspicious spirit falls in emptiness, good news is hard to rely on.