It is used for official orders or arrangments in the future. For example: "You are to clean your room before breakfast every day" "New recruits are to sign safety agreements before they begin working" This structure can also be used with the Passive Voice: "She is to be married on Sunday" "These forms are to be completed and returned to me before tomorrow" All of these examples suggest that the person doing the action did not choose freely to do it, but that it was arranged, necessary, or decided by somebody else. This structure is very formal and a little old-fashioned. It is more written than spoken, and I associate it with very formal language, for example in political or legal documents, or contracts. There is a good explanation here: http://www.grammaring.com/be-to-infinitive

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.

Thanks to all of you. Chris for trying to help, Fergus for a good link and explanation Su.Ki for clear explanation

Please provide some examples.