The Islamic calendar is based on the phases of the moon and so according to the authentic narrations of the Hadith of the Holy Prophet (pbuh), the best days for Hijama cupping are the 17th, 19th and 21st of the Islamic lunar month. Ideally these days should coincide with Monday or Tuesday.
Anas ibn Maalik (ra) reported that the Holy Prophet (pbuh) said, “Whoever wants to perform Hijama, then let him search for the 17th, 19th and 21st…” [Saheeh Sunan ibn Maajah (3486)].
Regarding having Hijama cupping performed just for Sunnah purposes, in his book ‘Al Afraad’, Al Daroqni related that Abdullah ibn Omar quoted the Messenger of Allah (pbuh) as saying, “Hijama increases ones memory and wisdom, apply Hijama in the name of Allah (SWT), but do not apply it on Thursday, Friday, Saturday or Sunday. Monday is the best.”
Regarding patients who are unwell due to a medical condition for which they are receiving Hijama treatment, they can be cupped on any day of the week and at any time of day.
Source: Ami Bint Aadam | HijamaforHealth | http://hijamaforhealth.blogspot.co.uk/