The best and most beneficial days to have hijama performed, also commonly known as the 'Sunnah Days' are the 17th, 19th or 21st of the Islamic (Lunar) month which fall on a Monday, Tuesday or Thursday.
Taking into consideration that in Islam the night enters before the day. So, at sunset on Tuesday, Wednesday night comes in and so on.
Hijama Therapy is more beneficial when performed during the daytime between Fajr and maghrib.
If you are not able to be cupping on the Sunnah days then the closest Monday, Tuesday or Thursday to the Sunnah Days are the next best days. Likewise, if there are no Sunnah days in a month, then the closest Monday, Tuesday or Thursday to the 17th, 19th, and 21st days of that month. If not, then on any Monday, Tuesday or Thursday of the month.
Below are some narrations of the Prophet (sallallahu alaihi wasallam) mentioning the best time to have Hijama performed:
Narrated Anas (radiyallahu Anhu), that the Prophet (sallahu alaihi wasallam) used to get treated with cupping on the two jugular veins and the upper part of the back. He used to get treated with cupping on the 17th and 19th and the 21st. (Jami’ at-Tirmidhi 2051)
Narrated Anas ibn Malik (radiyallahu Anhu) that the Messenger of Allah (sallallahu alaihi wasallam) said: Whoever wants to be treated with Cupping let him search for the 17th or the 19th or the 21st. And do not let your blood become hyperaemic (becomes excessive/boil/rage) so that it kills you. (Sunan ibn Majah 3486)
Narrated Abu Hurairah (radiyallahu anhu) that the Messenger of Allah (sallallahu alaihi wasallam) said: Whoever gets treated with Cupping on the 17th and 19th and the 21st will be cured from every illness. (Sunan Abu Dawud 3861)