Can I trigger "When I attack" effects any time in combat?

No, you cannot trigger "When I attack" effects at any time during combat. These abilities have specific timing requirements defined by the game rules.

According to Rule 376.4.d.2, "When I attack" triggers are put on the chain as pending items only after the unit gains the Attacker designation during combat.

Here is the breakdown of how these triggers function:

1. Trigger Timing: These abilities trigger specifically when a unit is designated as an attacker. This typically happens at the very beginning of a combat showdown (Rule 442.1.a.1).
2. Frequency: Per Rule 376.4.d.2.a, each unit can only fulfill its "When I attack" trigger once per combat, even if the unit gains and loses the Attacker designation multiple times during that same combat.
3. Late Arrivals: If a unit joins an ongoing combat late, it will gain the Attacker designation at that moment and its "When I attack" ability will trigger at that time (Rule 442.1.a.3.a). However, it still cannot trigger again if it has already triggered once during that specific combat.
4. Chain Interaction: Once triggered, these abilities are placed on the chain. Because they are on the chain, players can respond to them using Reaction speed spells or other triggered abilities, but you cannot manually activate or "force" these triggers to happen outside of the moment the unit gains the Attacker designation.

Summary of the Sequence:

• Combat Begins: Units are designated as Attacker/Defender.
• Triggering: "When I attack" abilities trigger and are placed on the chain.
• Reaction Window: Players may add reactions to the chain.
• Resolution: The chain resolves in LIFO (Last-In, First-Out) order.

If you have a specific card in mind, please provide the name, and I can confirm how its specific ability interacts with these rules.