First, take a very large tape measure...

Why do walnut trees produce walnuts, but oak trees produce acorns?

English is a combination of Danish with the original Anglo-Saxon, mixed thoroughly with Old French, spoken after 1066 by anyone in England who mattered, and studded with Latin, Greek and even some Celtic words. Seasoned with loan words from all around the world, it is no wonder that the seeds of Olde English Oak trees are called acorns, or maybe oakings, and walnuts are actually from walnut trees.

What is notable about the calendar dates 31st July-1st August, apart from the end of our local NarniaWeb competition?

It's the only time (within a single year) that a month switches over to another month which has the same number of days.
[Sorry if you were referring to something else; that was the only thing I could think of. ]

Why does it only seem to rain when you're having a picnic or some other outdoor event, and not when you're going to have to water the garden if it doesn't rain?

Don't you know that the best rain dances of them all are to wash your car, hold a picnic or barbecue or organise a game of golf? That way the rain will want to come to the party.

When will be the next blue moon?

When someone sings the song.

Why are moths attracted to a light source, yet don't usually come out during the day?

They do, they are just all flying towards the sun.

Why do leaves fall in the Autumn?

There is less sunlight for cold-climate deciduous trees to synthesize, so they downsize the amount of leaves to do it with.

What is a widget?

It's a distant cousin of thingamabob.

Why are dictionaries so large?

A dictionary covering every concept of the English language, in every dialect, shade of meaning, antonym, synonym and homonym would be big and heavy enough to squash any argument.

Why would a woodchuck chuck wood?

Perhaps because woodchucks like the taste or perhaps to sharpen their teeth.

What metal is most useful?

Heavy metal (the musical style), of course!

Red, purple, blue, green, yellow, white? (Bonus points (and possibly a virtual cookie) if you get the reference. )

Orange. (Sorry, I don't get the reference, though.)

What color is the loudest?

RED!!!

What vegetable is the most mischievous?

Cucumbers.

What type of dancing is the best?

The leap and caper of joy I will be doing once our tax is done and dusted for the year.

What is middle child syndrome, and why is it so prevalent?