What to wear
The hostess’s written invitation read “informal wear.” When I showed up I was the only one dressed informally. 
How can I avoid this happening again? 
Keep in mind Dominicans like to dress up. When in doubt, it is better to err on the dressier side. 
This question offers the opportunity to emphasize the most important guideline for any invitation: what is stated on the card is a cue for the gentlemen. The ladies should interpret that information by asking themselves three questions: (a) Where am I going? (b) What type of activity? (c) Who is doing the inviting?
Let’s look at the following examples for clarification.

Traje oscuro 
Traje oscuro literally means “dark suit.” This indicates a fairly formal affair, so right away you know you will have to dress up somewhat. You may, however, be unsure as to the exact degree. So ask yourself the three key questions to narrow down your options. 

If, for example, it is an invitation to an opening of an art exhibition and the card says traje oscuro or formal, you may wear a dressy outfit but nothing exceptionally fancy. 
On the other hand, the same thing may appear on an invitation to a reception at an ambassador’s residence. In this case, you would definitely get out your prettiest cocktail dress and treat yourself to a hairdo and manicure as well. 
Your escort would probably be able to wear the same thing to both events. (It’s not fair, is it?) 
The invitation which states “informal” as the dress, simply means the men do not have to wear suits and ties—they may wear informal trousers and sport shirts.

Traje de calle 
The literal translation is “street suit.” A dark suit is generally worn although a less formal one of a lighter color would be equally acceptable.

Traje formal 
The only time when a dinner jacket (tuxedo) is required is when the invitation mentions “smoking.” “Formal” means a dark suit, charcoal grey or black being the best, with an immaculate shirt and tie.

My daughter has been invited to a formal 15 year old birthday. What should she wear?
Choose a dress with a youthful style—for example, a fitted bodice with a short full skirt, but in a beautiful material such as raw silk, brocade for the top, lace, chiffon, organza in one of the wonderful jewel colors available these days. Keep the style simple. Short is the more popular length although a long skirt may be worn. 
If your daughter, like most teenagers, does not own anything more formal than blue jeans and a T-shirt, your best bet is the dressmaker. She can help you choose a suitable style and material and it really does not come out too expensive. 
Remember, a skirt with a great blouse is also very acceptable. Just do not miss enjoying this opportunity to celebrate a charming Dominican custom; if you are really desperate, you could even let her raid your wardrobe! 

I have been invited to a formal Dominican wedding. What are the options for men and women to wear?
There are seasons here, though not as marked as the ones with which you may be more familiar. For spring and summer, women should choose light fabrics such as chiffon, organza or voile. In the autumn and winter you may use darker hues in fabrics such as peau de soie, lace, velvet and brocade. Silk of any kind may be used all year round. 
Close note should be taken of the time of the wedding. A day time affair requires different attire from one in the evening. For the latter, you may dress “to the nines,” whereas for a daytime wedding a day dress or suit would be more appropriate. Note despite this being a tropical country, most Dominican women will wear stockings.
Hats? If you enjoy wearing them then by all means add a little glamour to the occasion; they are sold in the better stores. It is no longer obligatory for females to cover their heads in church nor, indeed, their shoulders. It is generally agreed that it is better to be overdressed than underdressed.
Your escort should wear a dark suit and tie for an evening wedding, a light suit being permissible for one in the day unless, of course, different dress is mentioned on the invitation.

How do Dominicans dress to go to the National Theater? To the cinema?
Again it is important to know more about the kind of event which you are attending. For a night at the symphony, gentlemen may wear a jacket and long sleeve shirt. No tie is necessary, but most men do wear one. The ladies wear day dresses, trousers or even simple cocktail dresses. There is very efficient air conditioning at the Theater and you should take a wrap or a jacket to avoid freezing. Jeans and bare feet are not permitted and you will be refused entry. 
If you are going to a play, the men may wear the typical Dominican chacabana (a jacket-type shirt), or a sport shirt and jacket.

Only when it is an “invitation only” benefit performance or gala event do you need to dress more formally. The gentlemen should then wear dark suits and the ladies, cocktail dresses. 

What should I wear to a funeral?
The important thing in this case is to dress discreetly. Avoid bright colors if at all possible. Dresses in subdued colors such as black, white, navy blue, grey, or cream will be worn by most women. A black skirt and white shirt with a pearl necklace is also suitable.