This is key from BBC Learning English Today.

I'm going to tell you about some informal ways of using English in English.

When we want to tell a friend about something we don't use the word notify.

That's what businesses do.

Businesses notify their customers and the prime minister might notify ministers.

But we don't notify our friends about our weekend plans.

It's too formal in English.

We say I'm going to let my friend know that I'm going out.

Let him know, not notify.

Let him know now, you know.