Once in a blue moon

 If something happens once in a blue moon, it occurs very rarely.
 "Bill has very little contact with his brother. They see each other once
 in a blue moon."

 Cloud nine

 A person who is on cloud nine is very happy because something
 wonderful has happened.
 "When the boss announced my promotion, I was on cloud nine!"

 Come rain or shine

 If a person does something come rain or shine, they do it regularly,
 whatever the circumstances.
 "He goes to the gym club every day, come rain or shine."

 In the dark

 If someone is kept or left in the dark about something, they are not
 informed about it .
 "The personnel were kept in the dark about the merger until the last
 minute."

 Reach for the moon

 If you reach for the moon, you are very ambitious and try to
 achieve something even if it's difficult.
 "Jenny is talented and ambitious; she always tends to reach for
 the moon."

 Chasing rainbows

 Someone who is chasing rainbows is trying to get something they will never obtain.
 "She's trying to get into Harvard but I think she's chasing rainbows."

 Teeth chattering

 If your teeth are chattering, you are extremely cold.
 "Was I cold? My teeth were chattering!"

 Face like thunder

 If someone has a face like thunder, they look very angry.
 "When Dad is really angry, he has a face like thunder!"

 Under the weather

 If you are under the weather, you are not feeling very well.