Celestial Dawn

If a nonbasic land has an ability that causes it to enter the battlefield tapped, it will lose that ability before it applies. The same is also true of any other abilities that modify how a land enters the battlefield or apply "as" a land enters the battlefield, such as the first ability of Cavern of Souls.

If a permanent enters while Celestial Dawn is on the battlefield, and then Celestial Dawn leaves the battlefield, the permanent's color will revert to the colors in its mana cost.

Land cards you own that aren't on the battlefield aren't changed to Plains.

If you have unspent nonwhite mana when Celestial Dawn enters, you still have that mana, but you can spend it only on the colorless part of costs.

Mana produced by spells you control and nonland permanents you control isn't affected by Celestial Dawn. Llanowar Elves will produce {G}, for example. However, any colored mana may be spent only on the colorless part of costs.

If you use a text-changing effect such as Mind Bend on Celestial Dawn to change the word "Plains" to a different land type, your lands are all of that new land type, and they will produce mana of the appropriate color for that type.