A round is a 50-day period during which the game takes place.
During the first three days (72 ticks) of the round or later if you join after the round has started, your dominion is under protection. Since the introduction of click-through protection this stage of the round no longer requires 3 full days to complete and can instead be done entirely at one's leisure between sign-up and the end of protection (OOP). Once the round has officially begun you must wait 24 hours after signing up to be able to participate in the round, unless you opt to begin using a Quick Start.
During the final day of the round (day 50),all offensive actions (including attacking and exploring) can be disabled anytime from 18h to 9h from the official end of round, at which point you are only able to train military, construct buildings, invest in castle, and perform info ops for the rest of the day. This is also the point of the round where players will typically reveal their locations and discuss the round openly since all outcomes are now locked in.