The Shulchan Aruch (545:2) discusses various restrictions that apply to writing during Chol Ha'mo'ed. As Chacham Bentzion Abba Shaul (Jerusalem, 1924-1998) notes, the Shulchan Aruch refers in this discussion only to writing that involves "Ma'aseh Uman," skilled craftsmanship. Normal, everyday writing with a pen, which is an elementary task that requires no particular skill or training, is permissible on Chol Ha'mo'ed, provided that it serves a practical purpose.
Thus, one may write a shopping list before going to the supermarket on Chol Ha'mo'ed so that he can remember to purchase all the items needed. Other examples include filling deposit slips or signing checks, writing down important phone numbers, and writing words of Torah. Writing is permissible in all these cases on Chol Ha'mo'ed.
Halacha forbids on Chol Ha'mo'ed only writing that serves no purpose, or writing in a skilled manner, such as the special writing of a Sofer (scribe). A Sofer may not practice his craft on Chol Ha'mo'ed, except to prepare a Torah scroll that a congregation needs for the holiday.
