Une periode charnière.
Charnière - Hinge
Let's start with tangible.
A jointed or flexible device that allows the turning or pivoting of a part, such as a door or lid, on a stationary frame.
And let's finish with less so.
A point or circumstance on which subsequent events depend.
I'll just come out and say it. I don't like hinge times. They're hard. All that pivoting or turning, being jointed or flexible on something stationary is difficult, even in the best of times.
In the worst of times, it's quite nearly superhuman. Which is how you feel, no doubt, when hinge time is over.
Either that or like a piece of gum that's been chewed up and spit out.
What are you hinging upon?