The words said and says are one of the two percent of English words that contain a sound that is a true exception! It is very unusual for ai to say /ĕ/, and it may be that the pronunciation of these two words shifted over time. As with most English words that are true exceptions, these are two very old English words.
When teaching the spelling of this word to students, we find it most helpful to show them spelling patterns found in other words:
It is important to teach the spelling of say and said using spelling analysis. However, it is helpful to show that their spelling follows a consistent pattern even though the pronunciation of one of the sounds is irregular.