a day-to-day activity is one of the things that you have to do every day, usually as a part of your work.

It's Sheila who's responsible for the day-to-day running of the school. (always before noun)

• the ball is in court
• the ball is in someone's court
• the ball is in your court
• the balloon goes up
• the bandwagon effect
• the bane of life
• the bane of your life
• the bare bones
• the battle lines are drawn
• the battle of the sexes
• the be-all and end-all
• the beau monde
• the beauty of
• the beauty of something
• the beginning of the end
• the best of both worlds
• the best of the best
• the best of the lot
• the best thing since sliced bread
• the better part of
• the better part of something
• the Bible Belt
• the big daddy
• the bigger they are, the harder they fall.
• the big picture