Good point from Alsacienne about holiday dates.

Greek Easter won't be a factor for the OP's trip as it's the same as our Easter this year - 20 April.

Ascension is on 29 May and the Pentecost public holiday on 9 June, so they won't be a factor either this year unless the OP is travelling at the very end of May. (Lots of Continentals take short breaks between Ascension and Pentecost)

The UK Bank Holidays are on Monday 5 May and Monday 26 May - and schools are on holiday for the whole week of 26 May.

So - most of May should be fairly quiet in all three countries.