Celestial Observatory is a Normal Spell Card.
Spell Cards are usually activated from the hand on your turn unless their type gives another timing window.
On the current TCG and OCG Forbidden & Limited lists tracked here, Celestial Observatory is Unlimited.
YGOPRODeck lists Celestial Observatory for these formats: TCG, OCG, Master Duel.
A listed print of Celestial Observatory appears in 2019 Gold Sarcophagus Tin Mega Pack (MP19-EN122) at Rare rarity.
Among markets shown on this page, the lowest listed snapshot for Celestial Observatory is about $0.14 on Cardmarket (prices change frequently).
