From Longman Dictionary of Contemporary Englishsomething is not to be missedsomething is not to be missedused to say that someone should do something while they have the opportunity A journey on one of the steam trains is certainly not to be missed! → miss
Examples from the Corpus
something is not to be missed• A tour of our wonderful capital city is not to be missed.• It is not to be missed.• Make a note of it: The Dish is not to be missed.• This show will be even better than the last one and is not to be missed!• During the day, the Acropolis is not to be missed and souvenirs can be bartered for in the Flea market. 