When I was wee, the First Sunday in May was an occasion for Catholic children to wear a new outfit - though I only remember girls talking about it. They'd get a new dress, hat, a bag, snowy white ankle socks, patent shoes. There was a lot of excitement and pride associated with it. Those who attended the kirk joined in a bit but lacked the fervour.
I'm not sure if the First Sunday in May is an official church festival, but I'll wear my new belt in commemoration of it anyway.