The Islamic day begins at sunset. This is particularity relevant to hijama in two ways.
(1) Lunar Days
Firstly, when calculating when the 17th, 19th and 21st days of the lunar month begin and end.
This month, for example, the 17th of the lunar month runs as follows.
(a) Saudi CALENDAR
Saturday 14 January 2017 at 4.23pm to Sunday 15 January 2017 at 4.24pm.
(b) Aqwaf Ministry, Maghreb
Sunday 15 January 2017 at 4.24pm to Monday 16 January 2017 at 4.26pm.
Secondly, when calculating when Wednesdays begin and end. We avoid where possible cupping on Wednesdays.
Particularly in the short days of winter in the UK we see that Wednesday begins earlier on a (Western) Tuesday at sunset and ends early on a (Western) Wednesday at sunset.
This week, for example, the Islamic day Wednesday runs from Tuesday 10 January 2017 at 4.16pm until Wednesday 11 January 2017 at 4.17pm.