Yes, Sunday voting is the norm in most countries and its the more sensible day to do it - people are off work, no need to close schools etc. A lot of countries that do have weekday voting do it on public holidays. The US is the other major outlier, theirs are in Tuesday.
I don't think anyone knows why we do them on Thursdays and I'm pretty sure that until recently it was convention rather than mandated. No reason why we couldn't have the EU elections on a Sunday like everyone else
They should try a Sunday and see if the turnout is any better for them.
If it was the first Sunday In May then we have the bank holiday the following day; easier for staying up to watch the results.
Ben Shatliff Online - There to help you