We define a trading day as a 24-hour period (10pm UTC - 9:59pm UTC) where at least 0.5% profit is made in that same 24-hour period.
The 0.5% refers to the amount of the starting balance of the stage you are trading.
For example, if you are in Level 2 of the 25K account with a balance of $50,000, then you need to make at least $250 in the 24-hour window for this to count as a trading day.