Answer:
"An holistic" is the wrong way to say it. You have to say "a holistic" because you only use "an" before a vowel (a, e, u, e, o). You use "a" before anything that is not a vowel.
Not exactly true. What about:
A user
A university
A unicycle
An hour
An honour
You would use 'an' before any word that begins with the sound of a vowel.