When is the traditional tide-watching day?

From August 16th to 18th of the lunar calendar (September of Gregorian calendar), the sun, the moon and the earth are almost in a straight line, so the tidal gravity (the difference between the gravity of the moon, the sun or other celestial bodies on the earth and the gravity of the earth's center of mass, or the resultant force of inertial centrifugal forces generated by the earth's movement around the earth's center of mass-the moon (the sun) and the moon (the sun)) is the greatest, which is the best time to watch the tide in Qiantang River.

So the answer is: September.