Day emptiness is the same as xunkong (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 xunkong (emptiness of a ten-day cycle). According to the ten-day cycle (xun) containing the divination day, the two Earthly Branches missing from that cycle are considered empty. For example, in the Jiazi cycle, Xu and Hai 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.