Mike Bickle introduces the Book of Joel, emphasizing its theme of the 'Day of the Lord' and the necessity for believers to respond appropriately to impending crises. He encourages attendees to read Joel multiple times and engage in discussions to deepen their understanding of its messages, particularly the call to repentance and the promise of the outpouring of the Spirit. Bickle highlights the dual nature of the Day of the Lord, which encompasses both revival and judgment, urging listeners to rend their hearts and seek God's favor through prayer and fasting. He asserts that the Book of Joel serves as a prophetic guide for the end times, equipping believers to navigate the challenges ahead with clarity and faith.