During the beginning phase, I hold at battlefield A with a unit equipped with Last Rites. Battlefield B is empty, and I play a Sneaky Deckhand there with my Last Rites hold trigger. Does the showdown start during the beginning phase, or in the main phase after I channel and draw for turn?

The showdown does not start during the Beginning Phase.

According to Rule 340, a Showdown begins when Control of a Battlefield is Contested and the turn is in a Neutral Open State. The Beginning Phase is a structured phase of the turn, not a Neutral Open State.

Here is the breakdown of why this occurs:

1. Trigger Resolution: When you trigger Last Rites during the Scoring Step of the Beginning Phase, you play the Sneaky Deckhand. This action occurs within the Beginning Phase.
2. Contested Status: While playing the Sneaky Deckhand to an empty battlefield makes that battlefield "Contested" (as you are attempting to conquer it), the game does not immediately transition into a Showdown because the turn is not in a Neutral Open State.
3. Staging: Because the battlefield is contested but the game state does not allow for a Showdown yet, the Showdown becomes Staged.
4. Showdown Initiation: Once you complete the Beginning Phase and proceed through the Channel and Draw phases, you will enter the Action Phase. The Action Phase is a Neutral Open State. Per Rule 322.13, if the current state is a Neutral Open State and one or more Showdowns are Staged, the Turn Player chooses one of those battlefields, and the Showdown begins there.

In summary, the Sneaky Deckhand is played during the Beginning Phase, but the resulting Showdown will not begin until you reach the Action Phase.