Since the main point of the tzitzit, as indicated by the verse cited above, is for the strings to be seen and to serve as a reminder, the mitzva of tzitzit applies only during the daytime, when they can be seen.

The obligation to wear tzitzit begins at the hour when there is sufficient daylight for one to recognize an acquaintance from a distance of two meters without artificial light. This is also the time of day when it becomes possible to distinguish between the white strings of the tzitzit and its sky-blue [tekhelet] strings. The timetables that list the daily halakhic times usually include the earliest time for each day when one may recite the blessing over the mitzva of tzitzit.

According to many halakhic authorities, the mitzva of tzitzit applies specifically by day. However, there is nothing wrong with wearing tzitzit at night as well. There are some authorities who hold that the mitzva applies even at night if one is wearing a garment that is worn commonly by day, and the mitzva does not apply to garments worn primarily by night, even if one wears them during the day. Accordingly, one who wears his regular garments of tzitzit at night would still be in fulfillment of a mitzva. Some people even go to sleep at night while wearing tzitzit, so as not to be separated from this important mitzva even for a moment.

As for wearing a tallit gadol at night, the kabbalists maintain that one should not wrap himself in a tallit gadol at night, whereas halakhic authorities have no objection to this practice. Each person should follow the instruction of his rabbi.

Once a year, on the night of Yom Kippur, it is customary for everyone to wear a tallit gadol even at the evening service.

Further reading: For more on wearing a tallit gadol, see the section dealing with the halakhot of Yom Kippur, p. 167.

In any event, the blessing over the tallit or tzitzit may be recited only in the daytime, when one fulfills the mitzva according to all opinions.