Sir Isaac Newton didn't discover his third law of motion on a specific date. He laid out his three laws of motion in his Philosophiæ Naturalis Principia Mathematica, published in 1687.

However, it's crucial to understand that Newton's laws are based on observations and deductions, not necessarily a single "Eureka!" moment. He likely developed his understanding of the third law over time, building upon the work of others like Galileo.

Here's how it likely unfolded:

1. Observation: Newton observed the interactions of objects. He saw that pushing a box forward caused him to move backward (think about pushing a heavy shopping cart). He saw how a cannon recoils when firing a projectile.

2. Deduction: From these observations, he concluded that forces always come in pairs. For every action, there is an equal and opposite reaction.

3. Mathematical Formulation: He formalized this principle mathematically, stating that whenever object A exerts a force on object B, object B exerts an equal and opposite force on object A.

So, while there's no specific date for the "discovery" of the third law, it was a process of observation, deduction, and mathematical formulation that took place over time.