You have the motivation to make a start with your ‘Why?’, you know ‘Who?’ you want to communicate with and you now know ‘When?’ and how much time you have to prepare. Now, ‘What’ do you want to say?
It never ceases to amaze me how much time is lost and wasted trying to learn grammar rules and phrases which have no relationship with the life of the learner. Even in school as children we are taught ‘the basics’ of languages very often without any connection to the lives of the children. I was lucky as a child because I was interested in how other people lived in other countries and I enjoyed learning their languages. Times have changed. I am sure that there are young people today who share my interest and also enjoy learning languages but they live in a different world from my 1970’s England. What shocks me most is that we still use the same system to teach languages to adults - years of study resulting in embarrassment caused by an inability to communicate in English at work. How frustrating! It simply makes no sense that somebody else decides what you should learn. Furthermore, repeating the same mistakes from childhood with grammar rules and the many rule breakers gets us nowhere. Knowing that the phrase ‘my father is a policeman and my mother is a nurse’ is written in the present simple tense is not very useful when you're trying to negotiate the price of machine parts - and even if your father is a policeman, who cares at that particular moment? It's simply absurd.
As adult learners we have to take responsibility for what we're going to learn. We should decide, not the author of the schoolbook. The book that you need hasn't been written yet, you have to do that yourself, with the help of a trainer or coach perhaps. So how do you do that? Well, you have seen how much time you have until your next event so you should focus on preparing for that. If it is a job interview then you need to prepare for talking about yourself, your achievements, your experience, your objectives, etc. If it is sales meeting then you need to prepare yourself to talk about products, services, prices, terms and conditions, delivery, completion, etc., and you're not going to find that in a schoolbook.
You can use your experience to plan your conversations and learn specific phrases and vocabulary that you need. You can write what you want to say in your own language and then try to put it into English. The ‘textbook’ you need is simply a notebook with blank pages for you to write in yourself. You can use a dictionary to find words that you don't know and you can use learning strategies to remember them. You might need some help to get the phrases right but that should be relatively quick and easy. Some audio recordings of the phrases will also be useful for learning the pronunciation.
The most important thing to focus on is the language you need for your next meeting or telephone call - if you need it once, you will probably need it again. Learn today what you need tomorrow or next week. Before you make a phone call write down some keywords or phrases that you want to use and plan your call, then follow your plan. If you have to make a presentation you can practice at home in front of the mirror - practice perhaps will not make it perfect, but it will certainly be better.
The more you practice, the better you will get.