It occasionally happens that a person will purchase the garment of a Tallit separately from the Sisit strings, requiring him to personally tie the strings onto the garment. The strings must, of course, be tied in accordance with Halacha, and with the clear intent that it is done for the purpose of the Misva of Sisit.
The question arises as to whether a Tallit is suitable for the Misva if the Sisit strings were tied during the nighttime hours. There is a Halachic principle that "Layla Lav Zeman Sisit" – the Misva of Sisit does not apply at night. Seemingly, then, if one affixes the Sisit strings at nighttime, when the obligation does not apply, the Tallit would be disqualified due to the rule of "Ta’aseh Ve’lo Min Ha’asui," which means that the Tallit must be suitable at the time the Sisit strings are attached. The classic example of "Ta’aseh Ve’lo Min Ha’asui" is a garment with only three corners, to which a person tied Sisit strings. If one would make a fourth corner in the garment, thus rendering it obligated in Sisit, and then affix Sisit strings to the fourth corner, the Tallit would be disqualified despite the fact that it has four corners with Sisit strings. Since the garment was not obligated in Sisit at the time the first three sets of strings were affixed, it may not be used. The first three sets of Sisit strings became potentially valid Sisit only retroactively, after the fourth corner was added. They are therefore disqualified due to the principle of "Ta’aseh Ve’lo Min Ha’asui," which requires that the Tallit be suitable at the time when one affixes the Sisit.
At first glance, this principle should apply also in the case of affixing Sisit at nighttime. Since the Sisit obligation does not apply until morning, it is only when the sun rises that the garment earns the formal status of a garment obligated in Sisit. The tying performed at nighttime should therefore be disqualified, in light of the rule of "Ta’aseh Ve’lo Min Ha’asui."
In truth, however, Halacha allows affixing Sisit strings to a garment at night, and such a Tallit is suitable for use. In the case of affixing strings to a three-cornered garment, the garment is intrinsically excluded from the Sisit obligation, and thus the strings attached to such a garment have no Halachic significance at all until the fourth corner is added. In the case of nighttime, the garment is inherently suitable, and we simply need to wait until morning for the obligation to take effect. In this case, what is required is simply the passage of time, rather than a specific action, to render the garment suitable to serve as a Tallit. Since the garment is intrinsically suitable, the Sisit are valid, even though they are tied at a time when the obligation does not apply.
This is the ruling of Hacham Ovadia Yosef, in his Yabia Omer, and of Hacham David Yosef, in his Oserot Yosef (Siman 35).
Summary: It is permissible to tie Sisit strings to a Tallit at nighttime, even though the Sisit obligation does not apply at night, and such a Tallit is perfectly valid for use for the Misva.
