"Your arrival is to happen any time before 5 o'clock" This means 'will happen' or 'is scheduled to happen'. The 'be to' form is an alternative to a future tense, mainly used for describing plans in formal and written contexts. And don't worry about duplicate questions - we get them all the time, and it doesn't really matter!

The construction "am/is/are + infinitive" is used to express an instruction in a firm but polite and formal way. e.g. "You are to check in at least 45 minutes before take-off". You could use "must" instead if you wanted to be more direct. I don't like the sentence you have found! I would never say this. This is because "to happen" is not an active verb and so does not suit this construction. I would change it to: "You are to arrive by 5 o'clock"