Customs of Wearing a Tallit
There are several times during the service when it is customary to kiss the corner threads symbolically:
Prior to the reciting of "Hear Israel" the corners of the prayer shawl are gathered together in one hand. At this time the corner threads should be checked to see that they haven't become unraveled or untied. If you have checked that the four sets of corner threads have five knots on each corner, you have done what is necessary.
Usually if there is a problem, it is that the last knot and some twists have come undone. The fourth and last section of the corner threads of each corner has thirteen twists and then a double knot. Correct what has come unraveled. (For further guidance refer to The Tying of the Threads of a Tallit)
During the recitation of the third paragraph of "Hear Israel" (Numbers 15:37-41) which mentions the threads three times, each time the word "threads" is read, it is customary to kiss the corner threads.
When the Torah is removed from the Ark and carried around the synagogue in procession, those within reach may touch the Torah mantle with the corner threads of their prayer shawl or with a closed prayer book, if they are not wearing a prayer shawl. It is then customary to kiss the corner threads or prayer book binding which touched the Torah scroll as an expression of love for the gift of Torah.
A prayer shawl is worn when making an Aliyah (blessing in front of the congregation before reading a section of the Torah portion). If you use a prayer shawl only when making an Aliyah, you needn't say the blessing.
If you borrow a prayer shawl for the service, say the blessing:
"Baruch atah adonoi, elohenu, melech ha olam asher kidshanu b'mistzvotav ve-tzevanu layitatef b'tzitit"
"Blessed are you, the Lord our God, ruler of the universe, who has made us holy with His commandments and commanded us to wrap ourselves in fringes."
A prayer shawl is not worn in the rest room.
If you take the prayer shawl off for a short time, you don't need to repeat the blessing when putting it on again.
The prayer shawl is worn for morning prayer during the week, on Saturday morning, and on other holy days. It is not worn for afternoon and evening prayers because of the commandment that one must see the corner threads and remember. (In ancient times, seeing depended on the light of day).
The three exceptions to these general rules are that a prayer shawl is worn at the following evening services:
Evening Kol Nidre service of Yom Kippur
Evening service of Simchat Torah
Special Friday evening services that include a Torah reading.
Rabbis and cantors wear a prayer shawl when conducting services except funeral services.
The leader of the prayer service (shaliach tzibur) wears a prayer shawl in the afternoon and evening as well.