Generally speaking, writing is forbidden on Hol HaMoed. The Mishna, however, lists various documents that can be written:
Deyateiki – Wills, and specifically the directive of someone who is on his deathbed
Perozbol (a document that turns personal loans over to the courts so that they can be collected after the Sabbatical year)
Igrot shum – Property estimates
Igrot mazon – Commitments to support adoptive children
Documents of halitza (release from a levirate marriage)
Documents of mi’un (refusal of a minor girl to a marriage arranged by her mother or brother)
What is the common thread that allows all of these different documents to be written on Hol HaMoed?
The Ri”d and the Ran suggest that these are not direct business documents, and since writing is not a difficult activity, the Sages were lenient regarding these cases.
According to the Rambam, all of these documents are essential for issues of public welfare, and they are permitted as tzorekh rabim – community needs.
Rashi, Rabbeinu Yehonatan, the Meiri and others argue that these are all situations of davar ha-aved – where potential loss will ensue if they are not committed to writing.
Why should a divorce, for example, be permitted?
Rashi understands the case to be one where a man is planning to travel on Hol HaMoed and is concerned lest his wife be left an aguna (literally “a chained woman” who cannot remarry) should he disappear; thus it is a davar ha-aved. Another suggestion is that in many cases of a contentious, unhappy home, a divorce is the only solution that will bring peace to the husband and wife, so it is considered tzorekh hamoed – necessary for the holiday. In a similar vein, the Talmud Yerushalmi asks why a sad event like a divorce would be permitted on the holiday, and responds that once a person has decided to divorce, writing the get – the divorce document – is not considered to be a sad event any longer.