Can you tap lands for mana before declaring attackers in your combat phase?

The combat phase is divided into five steps: Beginning of Combat, Declare Attackers, Declare Blockers, Combat Damage, and End of Combat. After the Beginning of Combat step begins, both players receive priority and may cast spells or activate abilities — including tapping lands for mana — before the game moves to the Declare Attackers step.

According to CR 505.1 and CR 506.1, the Beginning of Combat step starts, the active player receives priority, and only after all players pass priority in succession does the game advance to the Declare Attackers step. Tapping a land for mana is a mana ability (CR 605.1a) that can be activated any time you have priority.

It is worth noting that mana produced before attackers are declared does not persist automatically — your mana pool empties at the end of each step (CR 500.4). So mana tapped during the Beginning of Combat step will drain before the Declare Attackers step begins unless it is spent in that same step.

If you want the mana available during the Declare Attackers step (for example, to activate an ability tied to attacking), you should tap the land after attackers are declared but while you still have priority in that step — not before.

Example: You want to attack with Voltron Knight and then activate its {1}{W} pump ability after declaring it as an attacker. Tap your lands for mana after you declare attackers in the Declare Attackers step, not before, so the mana is still in your pool when you activate the ability.