By choosing to hold the wedding on a day when the sun is likely to shine, you have influence over the outcome, and the expected result is a wedding without rain

By choosing to hold the wedding on a day when the sun is likely to shine, you have influence over the outcome, and the expected result is a wedding without rain. It wouldn't be ironic for it to rain in Seattle in January on your wedding day. The theme of the song is that life has a way of teaching you lessons through irony by foiling your plans when you exert too much control. Man plans and God laughs, etc.