At a guess, absolutely nobody and its just a typo. Last year the elections were on the 5th May and the page hasnt been updated properly (they updated the year but didnt update the day)
No, it wont cause a revote. How many people ignore the date on their polling card, media coverage etc and consult this specific page on the SCC website to find out what date its on instead?
If you have genuine concerns, contact the Electoral Commission. Election fixing on behalf of whom? Which demographic/group of voters are more likely to ignore their voting card and look on the SCC website for the date instead, and thus miss the election? Show your working.