I think Dydd Gwyl Dewi (Saint David's Day) on 1st March in Wales and Saint George's Day on 23rd of April in England should be made to be bank holidays. I believe these dates are important for the people of the respected nations, and are a day of celebration that should be open for everyone to enjoy.
A national holiday for each nation of the UK is important and can be a day of celebration and reflection, this is something that already happens in Scotland, which celebrates Saint Andrew's day on the 30th of November. I believe it is therefore fitting for Wales and England to follow suit and celebrate their national day with a bank holiday as well.