tzitzis applies by day but not by night. The Rambam interprets this to mean that when a four-cornered garment is worn during the day it is required to have
tzitzis, but the same garment worn at night is not required to have
tzitzis. According to the Rosh, a "day garment" is required to have
tzitzis even if it is worn at night, but a "night garment" does not have to have
tzitzis when worn during the day. It is proper to be stringent and abide by both opinions; any garment worn by day should have
tzitzis, as should a day garment worn at night. A
beracha, however, is recited only on a day garment worn during the day.
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The obligation of tzitzis on bed linen
According to some poskim, only garments worn as clothing are obligated to have tzitzis. Blankets and the like, although placed on the body, are not considered clothing and are not required to have tzitzis. There is a stringent opinion that mandates even such items to have tzitzis. According to the Mishna Berura, linen used under the body for sitting or sleeping requires tzitzis, and in deference to this opinion one corner should be rounded when such items are made from wool. According to the Chazon Ish, however, such items are exempt from tzitzis and it is unnecessary to round a corner.
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When to recite the beracha of al mitzvas tzitzis
The earliest time to recite the beracha on tzitzis is from misheyakir - when it is light enough to distinguish between the blue and white strands of the tzitzis. This time can also be described as enough light to recognize a casual acquaintance from a distance of four amos. This time may be anywhere between six to thirty minutes after alos, depending on the various opinions. According to some poskim, a person who dons his talis between alos and misheyakir may recite a beracha. Other poskim hold that he should refrain from reciting the beracha until after misheyakir. At that time he should shake his tzitzis strings and recite the beracha. When possible, the second opinion should be followed. Someone who recited the beracha before misheyakir or even before alos should not repeat it after misheyakir.
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As a rule, women are exempt from time bound mitzvos-mitzvos aseh shezman gromo. Women have voluntarily accepted many of these mitzvos upon themselves. There is a machlokes as to whether women can recite a beracha when performing these mitzvos.
Women have not 'adopted' the mitzva of tzitzis and it is considered presumptuous for a woman to wear them. Additionally, the common types of talisos (gadol or koton) are considered men's clothing and therefore forbidden for women to wear.
A father is obligated to introduce his son to the mitzva of tzitzis once the boy is capable of dressing himself. There is an opinion that requires one to teach children about tzitzis once they reach the age when they learn to speak. The popular custom is to train boys to wear tzitzis from the time they are three years old.