Load the ENTSO-E day-ahead spot price (DE/LU) as an hourly series.
Reads column from interim/day_ahead_price.csv (written by spotforecast2_safe.downloader.entsoe.download_day_ahead_price) and converts the index to a UTC DatetimeIndex with hourly frequency. Missing values are rejected by default (fail-safe). The day-ahead auction price is published on D-1 and is leakage-clean at forecast time as long as the day-ahead value (not the realised price) is used.