Some people find it useful to practice mindfulness in the morning right after waking up since it sets a mindful tone for the day. Some people like to practice at the end of the day which may help with relaxing and falling asleep. Others schedule a time of day that works for them—for example, after finishing work, or after a workout, or after another regular daily activity. Some people practice at several different points of the day—for instance, when they awake, after finishing work, and before going to bed.
The key is consistency with the practice. Most people do better by picking and sticking to the time of day that works best for their schedule. If practice time is left to chance or done only when needed, then it is harder to go deeper into the practice and to use the skills when they are really needed.
Try making practice a part of the daily routine and commit. Then notice what happens on days when practice is done versus days it is not—perhaps this awareness will be motivating enough to keep practicing regularly!