Can I schedule a countdown timer to appear only when my sale starts?

Yes. In Products → Countdown Timer, fill in the timer as usual:

1. Choose Fixed end date and set Ends at to when the sale finishes.
2. Set Starts at (optional — hidden before this) to Friday 6 PM.
3. Tick Active and click Create.

Shoppers won't see the timer before the start time. It appears automatically at the start time and counts down to Ends at. When it reaches zero, the timer disappears from the page.

The timer doesn't change product prices. Set your sale prices on the products separately.