Hi Darling! I just adore weddings - as I adore any excuse to get all girlied up, but Spring weddings can be particularly challenging due to the weather. You never quite know if it will be drizzling rain or glorious sunshine. Also, even though the wedding is at 2:30 in the afternoon, don't be too casual - it is still a wedding after all. An afternoon wedding is still a dressy occasion, just not black-tie dressy.
It is perfectly appropriate to wear tights in March - but I would stay away from anything too dark. In fact, there are many really great patterned tights and sparkly tights that would work great with a shorter dressy dress.
The suggestion I have, however, is to go with one of Spring's hotter-than-hot long dresses. Choose something in a light fabric with a springtime feel to it, and dress it up with chic sweater or dressy jacket.
Finish with a pair of great heeled gladiator sandals and a gorgeous little handbag.
Another option is a dressy jumpsuit, topped with a sweater or blazer. Jumpsuits tend to be dressier than a basic trouser suit, and therefore perfectly appropriate for an afternoon wedding.