What is the appropriate attire to wear to an outdoor early evening wedding?

Assuming it will be during warm weather, a sundress and sandals for women (flats or wedges as heels will sink into the grass) and slacks and a dress shirt for men. If the ceremony is in a church, then the men should wear jacket and tie and remove them before the reception.

That would depend. It could be anything from blue jeans and a sweatshirt to formal attire.
It all comes down to what kind of wedding reception the bride and groom have arranged.

A clue might be what kind of financial circle the bride's parents travel in. If he's the president of a corporation... large or small, they might insist on formal.
Can you find out if the wedding is going to be with all the bells and whistles... bridesmaids, flower girl, and so on and so on, then you can assume a formal gown is expected; pant suits are out; men in tux or at least suits and ties.

Are you close to either bride or groom. Do you know or associate with anyone close to them. You could call up and ask.

You certainly don't want to show up looking like a penguin and be met at the door by a guy who's just stepped off his motorcycle.

I have been to two outdoor weddings this year. The first was small with only the parents, best man, and Matron of Honor. They just wore dress clothes and dresses. The second was a formal wedding with the Groom and party in tuxedos and the Bride in a Wedding dress and others in matching dresses. It really depends on what you would like to do.

I would wear a rather dressy sundress.

Something like these styles are nice

Spiegel | Product Page

Womens Career Dresses at metrostyle.com

I went to a back yard wedding in June and wore a black wrap dress ( I am from NYC and we always wear black) with silver wedge sandals. My date wore a shirt, tie and trousers. A summery sun dress with sandals would fit in even if it was casual. I would not recommend wearing high heels as they will sink in the grass