The Environment Minister sets the polling date.
Saturdays would seem to be more suitable unless of course they clash with major events, such as the controversy over the Six Nations Rugby Match against England in Twickenham back in 2016. The polling day was on the Friday before the match and politicians feared that a lot of their voters would be travelling to or already in England on polling day.
Some politicians have said that Friday is the best day for voter turnout and that for some reason Saturdays were not successful in this respect.