Advent Calendar--December 20

I drift into your ear sometimes
At first a beat, or tinkling tune
Some somber notes, some silly rhymes
The words some famous people croon

I stick without throughout the day
Humming in your head a bit
Accompany you on your way
To shop, to drive, to talk, to sit

I'll never leave your head, you know
Through seasons' changing, on and on
You'll hum me fast or sing me slow
The whole year round, at dusk or dawn.

What am I?