Making an overhand knot and opening such knots on Shabbos:
Two of the 39 prohibited Shabbos Melachos is Kosher and Matir, which is a prohibition against tying and untying knots on Shabbos. In certain cases, they tying or untying of a knot is under Biblical prohibition, while in other cases it is under Rabbinical prohibition, and in other cases is completely allowed. This is dependent on a) the amount of time that the knot is/was meant to last for and b) the form and stregnth of the knot.
A Biblical knot: Biblically one is only liable [on making] a permanent knot [i.e. Kesher Shel Kayama] which is made to last forever. Now, some Poskim rule that in addition to the above condition that the knot is meant to last forever, one is only Biblically liable for tying or untying the knot if it has been professionally done, which means that it is a strong and tough knot similar to the knots that the professionals make during their work. However, other Poskim rule that there is no difference between a professionally made and amateur made [knot], and one is Biblically liable for tying or untying any knot that is/was made to last permanently.
A Rabbinical knot: It is Rabbinically forbidden to tie or untie any knot that 1) is made in a professional manner even if not meant to last at all. 2) Likewise, it is Rabbinically forbidden to tie or untie any knot that is meant to last temporarily, meaning that one has in mind a set time by when he will undo the knot, and this set time is past 24 hours from when the knot has been made. 3) Likewise, it is Rabbinically forbidden to tie or untie any knot that is usually made to last forever even if one currently has no such intent.
Definition of professional knot: Any strong knot which is tightly tied there is doubt that perhaps it is considered a professional knot. Therefore [strong] double knots are forbidden to be tied or untied unless in a case of pain when made to not last 24 hours.
B. The law:
A single knot without a bow on top of it, is not considered a knot at all and is allowed to be tied and untied freely on Shabbos even if made to last forever. [However] this only refers to when tying two things together, however if one makes a knot using the single end of a rope or thread or string, then it has the same law as a double knot for all matters. [Thus, one may not make an overhand knot over a bag, such as a Challah bag on Shabbos. Likewise, one may not open such a knot, even if made from before Shabbos, and rather one is to break through the bag. Accordingly, it is proper to avoid making overhand knots over bags that will need to be opened on Shabbos. However, this only applies to a tight overhand knot, however, if the overhand knot is quite loose, such as if one did not tighten it, then it may be made and undone on Shabbos if its intent is/was to last for less than one day.]
 Admur 317:3
 Admur 317:1; Shabbos 73a and 74b
 Admur 317:1
 Admur ibid; Michaber 317:1; Mishneh Shabbos 113a
 Admur 317:1; Taz 317:1; M”A 317:1; Tur and Beis Yosef 317; Rashi 11b; M”B 317:1
Other opinions: Some Poskim rule that it is considered a permanent knot if it is made to last over a month, or over half a year or a year. [Beis Yosef 317 in one explanation of Rashi, brought in M”A 317:1; Rabbeinu Yerucham 14:12; See P”M 317 A”A 1]
 1st opinion in Admur 317:2; Rif 41b; Rambam [Shabbos 10:1] and Michaber 317:1
 2nd opinion in Admur 317:2; Rosh 15:1, Rashi, Tur; Rama 317:1
 Admur 317:1
 1st opinion and final ruling in Admur 317:2 “However a knot which is done professionally, even if it not made to last at all but rather to be untied that same day, is Rabbinically forbidden to tie or untie on Shabbos, due to a decree [that if this were to be allowed then one may come to make also] a professional knot that is meant to last forever of which one is Biblically liable on…. One is to be strict like the first opinion.”; Michaber and Rama 317:1; Rif 41b; Rambam Shabbos 10:1
Other opinions: Some Poskim rule that there is no difference at all between a professionally made and amateur made [knots], as even on an amateur knot one is Biblically liable if made for it to permanently last, and if [a knot] is designated to not last at all [then] in a situation that it is [thus] allowed to make an amateur knot it is also allowed to make a professional knot. [2nd opinion in Admur 317:2; Rosh, Rashi, Tur and Rama]
 Admur 317:1 “If one sets in his mind a designated time in which he will then certainly untie the knot, then even if this [designated time] is in a very long time [from now], it is not Biblically considered [a] permanent [knot] and he is [thus] exempt [from liability] whether upon tying it or untying it. However, Rabbinically this is also considered [a] permanent [knot] being that it is designated to last a certain amount of time and it is thus [Rabbinically] forbidden to tie or untie.”
 Admur ibid; 1st opinion in Rama 317:1; Kol Bo and Hagahos Maimanis; Taz 317:5
 Admur 317:10; M”A 317:20; Kol Bo 31; Yireim 274; Beis Yosef; M”B 317:34
 Admur 317:2 “However if it is not designated to last at all but rather to be untied the same day [that it was tied], such as knots of clothing and shoes of which when one places them on in the morning they are tied and when he removes them in the evening comes they are untied, then one is allowed to tie and untie it on Shabbos.”; Rama 317:1; Kol Bo and Hagahos Maimanis
 Admur 317:2; Michaber and Rama 317:1; Rif 41b; Rambam Shabbos 10:1
 Admur 317:2; Rama 317:1; Shiltei Giborim 41b
 Admur ibid; M”A 317:20; Hagahos Maimanis 10:10; Yireim 274
 Admur ibid; Rama 317:1; Semag
 Ketzos Hashulchan 123 footnote 4; Kaf Hachaim 317:22-23 based on 317: 2