Do you mean constructions such as "The conference is to be held next May"?
The answer is quite simple: it's one of our many future forms. You'll often come across this in formal writing. To be+to+infinitive is used to talk about a formal schedule or arrangement which has been set up. It can also be used to talk about what someone is supposed ( as in expected) to do: for example "According to this schedule, I'm to pick up the delegates at the airport at 11am".
In the past, this construction was common in daily conversation. For example, you might come across a character in a Jane Austen novel saying "Oh dear. What am I to do?". This means "What am I supposed to do?" or "What should I do?". You may even occasionally hear this kind of question today, as in "How am I to know how you're feeling? I'm not a mind reader". This is equivalent to "How should I know..?" in the sense of "How can I be expected to know?".
I hope that helps.