Ruling 1747
Leaving the place of iʿtikāf to perform recommended ghusls – such as the Friday ghusl or the ghusl for performing the rituals (aʿmāl) of Umm Dāwūd[1] – and similarly, leaving to perform a recommended wuḍūʾ, is problematic [i.e. based on obligatory precaution, one must not leave to perform them]. Generally speaking, leaving a mosque for supererogatory matters (i.e. those that are religiously preferred to be done rather than not done) – excluding matters that are commonly considered necessary – is problematic, and precaution must be observed [i.e. based on obligatory precaution, one must not leave the mosque for supererogatory matters]. However, a muʿtakif can leave the place of iʿtikāf for the purposes of attending a funeral procession (tashyīʿ al‑janāzah), preparing a corpse for ghusl, the funeral prayer (ṣalāt al‑mayyit), burial (dafn), and suchlike; [he can also leave for] visiting the sick and attending the Friday prayer (ṣalāt al‑jumuʿah).
-Ayatullah Sistani, Practical Laws of Islam, Spiritual Retreat (Iʿtikāf)
The rituals of Umm Dāwūd are a recommended set of acts of worship that are usually performed in the middle of the month of Rajab. See, for example, Shaykh ʿAbbās al-Qummī’s Mafātīḥ al‑Jinān, in the section on the recommended acts for Rajab.