A prophet is sometimes called upon to bear the heavy weight of a nation's history on his own shoulders. By performing a difficult, symbolic act of lying on his side for a prolonged time, the prophet physically illustrates and atones for the accumulated sins of the people. To make this burden possible for a single human being to endure, God mercifully condenses long eras of national wrongdoing into a set number of days. The prophet is instructed to focus on the years of the people's sin [מצודת ציון, ביאור שטיינזלץ, רש״י], with one day representing each year [ביאור שטיינזלץ]. The act of lying down serves as the method to atone for those years [מצודת ציון, מצודת דוד]. If the prophet were required to suffer for a full year to match every year of transgression, he would not survive the ordeal [רש״י, מצודת דוד]. Furthermore, using a day to symbolize a year is a natural comparison, as both represent a complete cycle of the sun [מלבי״ם].
The specific requirement to lie down for three hundred and ninety days prompts different understandings of Israelite history. The primary approach among commentators is that this number reflects the years spanning from the moment the Israelites entered the land until the exile of the Ten Tribes during the era of Hoshea son of Elah. This timeframe is calculated by combining the years the people were subjugated by foreign nations during the period of the Judges, which lasted one hundred and eleven years, with the reigns of the sinful kings of Israel following the division of the kingdom under Jeroboam son of Nebat [רש״י, מצודת דוד, רד״ק]. To reach the precise total of three hundred and ninety, some include the leadership of Samson and Shamgar, along with the era when Micah's idol was erected [רד״ק]. Another perspective suggests that the period begins with the initial sins of the Ten Tribes under Jeroboam and concludes later with the destruction of the First Temple [מלבי״ם].
Taking a completely different approach, [אברבנאל] rejects these historical timelines. He argues that the previous calculations are strained because they rely on the wrongdoings of individuals rather than the collective sins of the entire nation. According to his view, the target of this prophecy is actually the Kingdom of Judah, as the Ten Tribes had already been exiled. He explains that God commands the prophet to lie specifically on his left side, the side of the heart, to encourage deep, isolated contemplation about the ancient sin of the selling of Joseph. In this interpretation, the three hundred and ninety days, when combined with an additional forty days for Judah mentioned afterward, create a total of four hundred and thirty days. This number perfectly mirrors the four hundred and thirty years the Israelites endured in the Egyptian exile as a direct punishment for selling Joseph. The numbers are divided between Judah and the other tribes simply to highlight Judah's elevated status and importance.