Celestial Colonnade

This land enters tapped.
{T}: Add {W} or {U}.
{3}{W}{U}: Until end of turn, this land becomes a 4/4 white and blue Elemental creature with flying and vigilance. It's still a land.

Edge of Eternities: Stellar Sights

Doctor Who

A land that becomes a creature may be affected by "summoning sickness." You can't attack with it or use any of its {T} abilities (including its mana abilities) unless it began your most recent turn on the battlefield under your control. Note that summoning sickness cares about when that permanent came under your control, not when it became a creature.

When a land becomes a creature, that doesn't count as having a creature enter. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters won't trigger.

If Celestial Colonnade can't attack unless you pay a cost that includes a mana payment, you may attack with it and tap it to pay for that cost. It will still attack in this case.

Once Celestial Colonnade has attacked, tapping it for mana won't remove it from combat.