a peaceful and quiet period before a period of activity or trouble.

The family are arriving this afternoon so I'm just sitting down with a cup of coffee, enjoying the calm before the storm.

• at any rate
• at a pinch
• at a premium
• at a price
• at a push
• at a rate of knots
• at a snail's pace
• at a stretch
• at a time
• at beck and call
• at best
• at convenience
• at cross purposes
• at cross-purposes
• at death's door
• at each other's throats
• at earliest convenience
• at ease
• at elbow
• a tell-tale
• a tempest in a teapot
• at every turn
• at expense
• at fingertips
• at first blush