When are trailers released?

It is imperative that a trailer is released at the appropriate time, in order to get people excited about it, right up until its release date. It has to be released early so that there is enough people interested in watching the film, but not too early that they forget or get bored about it before it's even released. The opening weekend is when most people watch the film, so the advertising campaign has to be coordinated with this date. After this weekend, the buzz usually dies down, so everything up until this date is important.
Key release dates are usually during significant times of the year, such as; the summer holidays, Christmas, valentines day and school holidays. This is because most people are off work during these times, so they therefore want entertainment and have more time to relax. The release date also depends on the type of audience that the film is aimed at. For example, a chick flick like 'PS I love you' would be released on valentines day because it is about romance, which fits with the time of year. A horror film like 'Ouija' would be released around Halloween, because it is aimed at teens and that time of the year is focused on horror. Therefore the trailers for these movies would be released near these dates, in places that relate to the campaign. For example, a poster for a chick flick may be advertised near a place that sells chocolate and flowers for valentines. Or they might include a gimmick like 'treat your girlfriend this valentines day'.
Certain months like January, February, September and August are often called 'dump months' because this is when more obscure and less successful films are released. Because they aren't expected to generate much revenue, the producers wouldn't have a large budget for the advertising campaign. They would probably release the trailers nearer the time of the movie's release date since it's more likely that viewers would forget about it.
Many factors cause this decline in the entertainment industry during these 'dump months'. For example, September is when school starts, so people have less time to watch films due to these responsibilities. The advantage of this however, is that there is little competition since the more established films are released during other months.
Friday is the most common day of the week that films are released. On the other weekdays, most people, they have to get an early night for the sake of getting up for work/school in the morning. But on Fridays, they have the luxury of staying up late and ordering a takeaway since they have the weekend to fall back on.
Factors that distributors take into account when releasing a trailer include;

• The timing of the trailers release. As stated before, an early release risks that the audience forgets about the movie; but a later one may not give enough of a gap before the opening weekend to gain a large following. They should also ensure that the target audience will be interested in the movie at that time of the year. Holidays are ideal because there's no work/school to distract them.
• When the target audience is most likely to see it and get excited for it. During the Christmas holidays for example, families would be actively seeking out Christmas movies to watch on the day. This is a good time to advertise a Christmas or a snow themed movie.
• Competition. It is bad for 2 trailers of the same genre to be released at once, especially for box office films. For example with an action movie, one is likely to overshadow the other and take away attention from it, particularly if it gains more hype. On the other hand, if an audience is anticipating the release of a superhero movie, they'd probably be aware of other superhero movies that are also being released near the same time. In fact, a bit of competition and comparing between the films will bring them both publicity.
• When other countries are releasing the trailer. It is better to release them around the world at similar times, because having more people hyped at once will make the movie more popular. In this day and age, people from many different countries share things via social media and the internet. If a trailer is released in the US, it is likely that it will be shared in the UK. There is no point in releasing the trailer months in the UK months after the US, because by then, the hype will have died down. So the campaign has to be coordinated in all places.  

According to 'Pearl and Dean' (http://business.pearlanddean.com/films_trailers), some of the latest trailers to be released right now, include 'Beauty and the Beast' 'The BFG' and 'Ice age'. They are being released at this time of the year because they are aimed at kids and it is a couple of weeks before the summer holidays. This gives them just enough time for kids to be excited about the films and ready to watch them when school is over. The time between the trailer's release and the opening weekend is quite short because kids are more likely to forget about the film in a longer period of time.
Other successful movie trailers like 'Ghostbusters' and 'Doctor strange' are being released right now, for the same reasons. They are targeted at an older audience (like teenagers and young adults), but the timing of the summer holidays still applies here. This is when families spend time and sit down together; which is an important factor when creating awareness for the film.