Luminesce prevent damage black sources red sources magic

[peanutbutter]

Do I have to play Luminesce at beginning of turn before any damage is dealt or can I play it after damage is dealt?

[Arix]

Neither. The best time to play Luminesce is just before damage would be dealt - for example, in response to your opponent casting Lightning Bolt, or after your opponent has attacked but before damage is dealt.