When you hover over the text "on Friday" you get the clue "ar an Aoine". When you type this clue into the checkbox - an error occurs. Why?
Because the clue isn’t accurate — ar an Aoine is better understood generally as “on Fridays”. It could be translated as “on Friday” as part of a date phrase, e.g. ar an Aoine 13ú Feabhar 2015 (“on Friday, 13th February 2015”) because day names without Dé take an article.