Solidarity is a Continuous 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, Solidarity is Unlimited.
YGOPRODeck lists Solidarity for these formats: Duel Links, Common Charity, Edison, TCG, OCG, Master Duel.
A listed print of Solidarity appears in Ancient Prophecy (ANPR-EN054) at Rare rarity.
Among markets shown on this page, the lowest listed snapshot for Solidarity is about $2.36 on Cardmarket (prices change frequently).
