Different seasonality length
Does Prophet support two different sizes of seasonality?
I have data with this season length: [...,21,21,21,21,21,28,21,21,....]
But train results look like: [...,21,21,21,21,21,28,14,21,....]
Seasons setup:
model.add_seasonality(name='21_season', period=21, fourier_order=10, condition_name='21_CAMPAIGN', mode="multiplicative") model.add_seasonality(name='28_season', period=28, fourier_order=10, condition_name='28_CAMPAIGN', mode="multiplicative")
Results:

The typical seasonality for business metrics is a combination of day-of-year ("yearly") and day-of-week ("weekly"), which Prophet usually captures without issues.
In this case it seems you want to have different seasonality patterns for different parts of the training data, which is technically easier compared to having a combination. Just checking that the 21_CAMPAIGN and 28_CAMPAIGN flags are non-overlapping?
Yes.