Some quick thoughts - unlike excel’s cell by cell approach, Airtable runs its formula once per row. Meaning if you need to reference one row from another, you’re either using linked records or making use of a script instead of a formula.
But you might be able to get away with grouping and the summary bar. This is the simplest solution.
Combine days into the same row together. Then using lookups and formula fields, you’d calculate their averages however suits you.