Time series forecasting for unequal length data per year I wanted to forecast student fall enrollment using last 3 years of data. Registration open day for fall differs by year. For instance, Fall registration open day in 2019 was March 10 and ended in say August 22, March 20 in 2020 and ended in August 22, so on! Therefore, for 2019 I have total 155 days of data, 156 for 2020, and 165 for 2021. As a result, we have ended up with different length of time series per year. I wanted to forecast the fall enrollment for 2022 at the last day of registration. I am not sure how can address this issue! Most time series I have seen has equal time length per year. I appreciate your suggestions. Thanks!
Original data plot:

Sample data:


I have made a equal length time series here and fit ARIMA model to forecast the enrollment.
