The Forex market is open around the clock, Monday through Friday. Traders can place their trades at any time they want since there is always an open market somewhere in the world. When New York sleeps, Tokyo and Sydney are awake, and vice-versa.
However, despite the ability to place trades around the clock, certain times offer better profit opportunities than others. This is especially true for day traders and scalpers who open a large number of trades during the week or day and who can easily get affected by wider spreads or slippage.
Again, a good trading coach should assess your trading style before giving you a definitive answer to this question. Day traders and scalpers should focus on the most liquid time of a trading day, which is the overlap of the New York and London session. Swing and position traders, on the other hand, are not affected by wider spreads since they hold their trades for a longer period of time and have higher profit targets.