Is expiation obligatory for each day Tahajjud prayer was missed, or is it a single expiation for all days, and can this oath be annulled?

Qiyam al-Layl (night vigil prayer) is a recommended supererogatory act. If a person swears an oath to perform it, then abandoning it becomes reprehensible. The oath is dissolved by leaving it once, and expiation becomes obligatory, which is: feeding ten poor people or clothing them, or emancipating a slave. If one cannot find any of these, then fasting for three days. After the dissolution of the oath, the virtue of Qiyam al-Layl remains as it was originally.