Celestial Dawn is a special Enchantment card from Time Spiral Timeshifted (TSB), collector number 3. Current market price: $0.56 (foil $3.95), TCGplayer market prices via Scryfall.
Lands you control are Plains. Nonland permanents you control are white. The same is true for spells you control and nonland cards you own that aren't on the battlefield. You may spend white mana as though it were mana of any color. You may spend other mana only as though it were colorless mana.