Rabbinic courts do not convene on Shabbat or Yom Tov out of concern that the court would need to write down the claims of each side, thus transgressing a Torah prohibition (Beitza 37a). However, courts do convene on Ḥol Ha-mo’ed, and hear all cases, because normal writing for the sake of a mitzva is permissible on Ḥol Ha-mo’ed, and for a time-sensitive mitzva even skilled writing is permitted. Despite the mitzva to rejoice on the festival, the courts are still allowed to hand down rulings. In the past, when the courts had the power to do so, this included carrying out sentences of lashes and even the death penalty. This served the communal need to establish a just society and uproot wickedness from the Jewish people. In addition, in order to avoid delaying the atonement of the sinner that court punishments provide, they were administered on Ḥol Ha-mo’ed (MK 14b; SA 545:10).
Nowadays, although a rabbinic court neither hears capital cases nor administers lashes, it may hear cases which need to be dealt with on Ḥol Ha-mo’ed. Therefore, divorce documents, ḥalitza documents, and financial settlements may be dealt with then. Monetary disputes may be resolved and their resolutions written up. The two sides involved in a case may introduce preliminary materials, and their positions may be written up. If two sides agree to present their case before a specific beit din, a document to this effect may be written up and signed as well. If a borrower has not repaid his debt, his property’s value may be assessed. The lender may then be given his due in property, and a document may be written up to attest to it. This ensures that people will be aware that ownership of the property has been transferred to the lender. Child support provisions are written up on Ḥol Ha-mo’ed, as are wills and documents gifting people with property (MK 18b; SA 545:5).
The reason these are all allowed is because they meet communal needs. Even though each case deals with private individuals, such cases come up frequently. If dealing with them is delayed until after Ḥol Ha-mo’ed, it is possible that in the meantime the two sides will resume fighting. Avoiding this is a communal need (MT, Laws of Yom Tov 7:12; Responsa Rivash §390). If necessary, even skilled writing may be done. The justification for doing so is that sometimes writing down settlements is a davar ha-aved, since if they are not written down immediately the sides are likely to renege on agreements already reached (Tosafot). Occasionally, writing down these agreements actually pertains to a bodily need, as doing so may mean that one will be able to collect the money needed to buy food for the festival meals (Pri Megadim, Mishbetzot Zahav 545:7; MB ad loc. 20; see above 11:13).
For these reasons, a bank may open on Ḥol Ha-mo’ed. True, le-khatḥila it is preferable not to do banking on Ḥol Ha-mo’ed, barring great necessity. Therefore, banks should reduce their Ḥol Ha-mo’ed hours of service. Similarly, their clients should refrain from transactions which can be delayed until after the festival. However, when necessary, clients may use the bank, such as to deposit checks that otherwise might get lost or might expire, to withdraw cash for festival expenses, or to pay bills that carry late fees (SSK 68:2).
Similarly, postal workers may open the post office, because it serves a communal need. There may be letters in the mail whose late arrival may cause a loss. Postal workers may do skilled labor if necessary.
It is a Torah commandment to lend money to one who needs it, and it is a mitzva to document the loan. The documentation is important as it avoids any future disagreements about the amount or terms of the loan due to forgetfulness or dishonesty (SA ḤM 70:1). However, it is preferable not to borrow money on Ḥol Ha-mo’ed, in order to avoid the need to write a loan document then (SA 545:6). Nevertheless, if one must take out a loan in order to buy festival necessities or to avoid a loss, one may give him a loan and document it. If necessary, the writing may even be done by a professional (ibid.; MB ad loc. 36-37).