Every seven years, Israel was to observe which type of year, where the fields were left unplowed?

The Sabbatical year, known as "Shmita" in Hebrew, is a practice detailed in the Old Testament in which every seventh year, the fields were to lie fallow. This meant that agricultural work was to be suspended, allowing the land to rest and rejuvenate. During this year, Israelites were instructed to not sow their fields or prune their vineyards. Instead, whatever the land produced naturally was to be available for the poor and for the wild animals. This observance highlighted the importance of rest and trust in God’s provision, ensuring that both the land and the people could recuperate.

The Jubilee year, though also significant and occurring every fifty years, focuses on liberty and the return of properties to their original owners, rather than the agricultural practice of leaving fields unplowed. The concept of a harvest year or holy year doesn't encapsulate the requirement for a fallow year as explicitly as the Sabbatical year does within the covenantal laws given to Israel. Thus, the Sabbatical year is specifically designated for this practice, reinforcing the theological principles of rest and social justice inherent in God's covenant with His people.