When do defender and attacker get designated? When combat starts or when showdown starts?

Attacker and Defender designations are assigned when Combat opens — specifically as the first task of the Combat Showdown Step (Rule 459.2.b). They are not assigned when a general (non-combat) Showdown begins.

Here's the distinction:

• Non-Combat Showdown (conquering an empty/uncontrolled battlefield): No Attacker or Defender exists. There's a Showdown for response opportunities, but it never enters combat — no designations, no combat damage, no combat cleanup. (FAQ #6910; Rule 344.2)

• Combat Showdown (enemy units are present when Contested is applied, or Combat is staged): As soon as Combat opens, Step 1 establishes who is Attacker and who is Defender. The Attacker is the player whose unit applied Contested, and the Defender is the other player. Their units at that battlefield gain the matching designation at that moment (Rule 459.2.b.1–4).