u/Different-Mud-5926

So i saw the card [[Cemetery Illuminator]] and it says "Once each turn, you may cast a spell from the top of your library...". But other cards like [[Invoke Calamity]] says "You may cast up to two instant and/or sorcery spells...".

I am just wondering when you are allowed to cast spells outside timing restrictions. I saw something about the "once each turn" part giving a duration to cast it and invoke calamity happening during the spell resolving. I am just not sure about the exact ruling, when can i cast stuff at instant speed and when can i not.

Also [[Mosswort Bridge]] is the same, but this one says "You may play the exiled card". I know lands are played, but you cant play a land at instant speed right?