Some days are just fun.
The story goes that after his death when the monks tried to move his body from a simple grave into a gold shrine in the cathedral, he started it raining for 40 days to discourage it. I presume they gave up.
This saint reminds me of my sainted Grandmother. When we went to send my grandmother off, it was already raining, but it stopped every time we needed to do something outside. Once we had buried her in the ground it began to rain, presumably to drive us away and back to our lives.
Sure sign that both have influence in Heaven.
Saint Swithun's day, if thou dost rain,
For forty days it will remain;
Saint Swithun's day, if thou be fair,
For forty days 'twill rain nae mair.
-- An old rhyme