Should the zakat on the plot of land I bought for trading be paid in Dhu al-Hijjah, or with my usual annual zakat payment in Sha'ban, knowing that if I had not bought it, I would have paid zakat on its value in the current Sha'ban?

The time when Zakat becomes due on trade goods is the same as the time Zakat became due on the capital used to purchase them, provided it met the nisab. Therefore, the Zakat on the land is due in the month of Sha'ban, calculated based on the land's value in Sha'ban, and one-quarter of one-tenth (2.5%) of this value is to be given.