Be on your best behaviour
Definition 1 of 1
Pro Tip 1/3Special Occasions
Used during events where polite behavior is expected, like weddings or formal dinners. 
She knew to be on her best behaviour during the awards ceremony.
Pro Tip 2/3Expectation
Suggests that there's an expectation to act better than usual. 
The children were reminded to be on their best behaviour when the guests arrived.
Pro Tip 3/3Impression Matters
Important when first impressions count, like meeting someone's family. 
He was on his best behaviour when he met her parents for the first time.