What does "Gu Erdan" mean on the tombstone?

A tablet marking the auspicious days of the ecliptic in a certain period. In other words, people will be asked to find out when and when it is a good time to build a monument. For example, "Moon Valley Dan in the 35th year of the Republic of China" refers to a monument built by Ji Hitachi in Mongolia. Another example is "Ding Si, who is two years old, will go to beginning of autumn Middle School in Gu Huan", which means the same thing. When explaining Memorial Day, many tombstones will use the words "Gu Dan", "Ji Dan" and "auspicious day". The word Gu Erdan comes from The Book of Songs, Chen Feng, East Gate. There is a saying in the poem: "Gu Erdan is poor, the source of the East". "Biography of Mao" explains: "The valley is good." Jian Zheng explained: "Dan, Ming." Therefore, the word "Gu Erdan" on the stone tablet means a beautiful day. Besides "Gu Erdan", some tombstones also use words such as "Ji Dan" and "auspicious day", which have the same meaning as "Gu Erdan". The use of these words, in addition to the pursuit of literal good omen, may often involve superstition of good words or bad words.