Hey, so lets say I'm forecasting some sales based on biweekly data (the timestamps are the end of the 2 weeks) and I have a column in the dataframe called "holidays_in_2_weeks", which basically is the count of the holidays in that time span of every entry. Would I pass this as a hist_exog_list or a futr_exog_list? And can you give me reasoning too? Thanks!

I would passs it as

futr_exog_list

, since you can easily know how many holidays there will be in the future and that information can be used to inform the predictions.