Reference: Fataawa Ramadhaan – Volume 1, Page 893, Fatwa No.880
Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.8701
Question: Is it permissible for one who wishes to perform the i’tikaaf to specify a particular day itself for i’tikaaf?
Response: It is not for him to specify a particular day itself within which to regularly perform i’tikaaf, rather, he should take care to perform i’tikaaf in the last ten [days and nights] of Ramadhaan in accordance with [the guidance of] the Prophet ﷺ.
And with Allaah lies all success, and may Allaah send prayers and salutations upon our Prophet Muhammad ﷺ and his family and his companions.