סימן קד סעיף טו

חֶשְׁבּוֹנוֹת וְכַיּוֹצֵא בָזֶה, שֶׁאִם לֹא יִכְתְּבֵם, יִשְׁכָּחֵם, מֻתָּר לְכָתְבָם, מִשּׁוּם דַּהֲוֵי דָּבָר הָאָבֵד. וְכֵן מַה שֶּׁהוּא לְצֹרֶךְ הַמּוֹעֵד, מֻתָּר לִכְתֹּב. אֲבָל שְׁאָר דָּבָר, אָסוּר לִכְתֹּב. וְאִגֶּרֶת שְׁלוֹמִים שֶׁכּוֹתֵב אָדָם לַחֲבֵרוֹ, נוֹהֲגִין לִכְתֹּב בְּשִׁנּוי קְצָת, דְּהַיְנוּ שֶׁכּוֹתְבִין שׁוּרָה רִאשׁוֹנָה עֲקֻמָּה. וּלְכָל מַה שֶּׁמֻּתָּר לִכְתּוֹב, מֻתָּר גַּם כֵּן לְתַקֵּן קֻלְמוּס וּדְיוֹ.

It is permitted to record bills and similar records which, if not recorded will be forgotten because it is considered prevention of a loss.⁴¹Shulchan Aruch 545:4. You are permitted to write⁴²This, however, is only permitted if the writing is commonplace and not professional. (Mishnah Berurah 545:18) whatever is necessary for the needs of Yom Tov, but it is forbidden to write anything else. Social letters that friends write to each other, should be written with a slight change,⁴³Rema 545:5. However, you may not purposely leave your letter writing for Chol Hamoed. (Shibolei Haleket, Mishnah Berurah 545:31) The reason some permit writing letters to a friend is because it makes them happy and is, therefore, considered a need of Chol Hamoed. (Ritva, Peri Megadim, see Sha’ar Hatziyun 545:30) This would mean the letter would have to be received before Yom Tov is over. It is best not to write about things pertaining to business. (see Biur Halachah 545:5) like writing the first line unevenly.⁴⁴Bach, Taz etc. But Magein Avraham says to make all the lines uneven. For everything that you are permitted to write, you are also permitted to prepare a pen and ink.

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)