You can practice yin yoga at any time of the day, depending on your goals:

In the morning, the muscles are not yet warmed up, which means they will not take on most of the load, as usual. This will allow you to work deeper into the joints and ligaments. Moreover, if during the day you plan to perform an even more intense “yang” training, due to the preliminary deep work with the connective tissue, the load from physical exercises will be distributed more evenly, which means that strength training will bring more benefits.
During the day, yin-style practice will also be appropriate. But here, how active you are during the day plays a role: if you sit at the computer all day, the Yang approach will bring much more benefit. The motor load on the muscles will improve blood circulation and tissue trophism, you will feel a surge of strength and vigor after class.
In the evening, the muscles are more active: it will be more difficult to let go of control and get to the connective tissue. But Yin Yoga will perfectly reboot the nervous system, set it up for rest and deep recovery. This is especially important for people who experience stress or anxiety during the day.
Yin practice is especially beneficial for women during menstruation.

In terms of seasonal features, summer and spring are considered the most active, or yang, seasons of the year. Therefore, if you spend a lot of time on the street, travel, play sports, then in order to maintain inner balance this season, it is especially useful to engage in Yin practices. But in autumn and winter, when there is less activity and shorter daylight hours, it is worth focusing on active Yang practices.

In a word, when choosing a time for practice, focus primarily on your lifestyle and level of daily activity. The more movement, active training, emotions, impressions, the more necessary Yin practice is to maintain balance. Moreover, during such periods, you can perform it daily: connective tissue does not need as much time to recover as muscle after intense strength work.

As a rule, we often choose the types of practices that we like best, but this is not always to our advantage. So, people who are calm and slow prefer unhurried yin-style practices, while energetic and active people prefer dynamic training. If your goal is health and balance, you need to learn how to balance both approaches: “dilute” hot yang practices with unhurried yin yoga and vice versa. This will allow you to avoid extremes, maintain the health of all types of tissues and optimal energy levels.