Ideas on a Forecasting Problem

​ Hi everyone, I'm working on a retail/e-commerce forecasting project where we need to predict synthetic demand (actual sales + lost sales due to stockouts) during peak festival times. We are trying to calculate the lost demand when an item goes Out of Stock (OOS), but the extreme volatility of the short festive window is making standard historical imputation impossible. The Data We Have: Periods: Last Year BAU, Last Year Festi