EX raids only during weekdays ?

Ex raids are on one set day for 3-4 weeks and then move ahead one day, so eventually, they'll be on weekends.

They do indeed. For a while, EX raids followed a progression that eventually covered the entire week, e.g. you'd have a couple on a Monday, then couple on a Tuesday etc.

However, in recent weeks, it's been a bit unpredictable, jumping from Friday to Wednesday in our town.

Not worth trying for, IMO. I’ve skipped dozens and dozens of EX raids because on average they occur Tuesday at 10:30am, or a comparable inconvenience.