פירוש על שבת 20:1

רבי אליעזר אומר: תולין את המשמרת – they filter in it the sediment of the wine and stretch/extend its mouth over the entire side in a circle and it makes a “tent” over the hollow/cavity of the utensil but even though he made a “tent,” it is permissible on Jewish holy days, for Rabbi Eliezer holds that acts preliminary to the preparation of food where it is possible to do them from the eve of the Jewish holy day, it is permitted to do them on the Jewish holy day, but on the Sabbath, ab initio, he may not suspend [a strainer] , but if it is [already] suspended, he may place into it the sediments, and stretch it for this is not the manner of "בורר"/sifting (which is one of the primary forms of labor prohibited on the Sabbath).

Introduction This mishnah deals with pouring wine through a strainer on festivals and Shabbat. They would suspend or sort of stretch the strainer over the mouth of a vessel and the unstrained wine would be poured through it. In order to understand the mishnah we must remember that on festivals food may be prepared, even if this requires a labor normally prohibited such as slaughtering an animal or cooking a dish. The issue discussed here is can one prepare vessels with which to prepare food if these vessels could have been done prepared before the festival. On Shabbat there are two issues involved in this mishnah: 1) the preparation of food; 2) the making of a vessel.

אין תולין את המשמרת - for he is making a temporary tent and it appears like an act of the weekdays.

R. Eliezer says: one may suspend a strainer on festivals, and pour [wine] through a suspended [strainer] on Shabbat. According to Rabbi Eliezer, even though she could have set up the strainer the day before the festival, since the strainer is set up in order to prepare food, which is permitted on festivals, she may also suspend it on top of the vessel on the festival itself. She obviously may pour wine through it. On Shabbat, she cannot suspend the strainer for this is considered similar to making a tent, because she is placing the strainer on top of the vessel. However, Rabbi Eliezer holds that it is permitted to pour the wine through the strainer if it is already suspended and this is not considered an act of “selecting” (see 7:2). “Selecting” is separating the pebbles from the kernels and this is not similar enough to be forbidden.

ואין נותנין לתלויה בשבת – for it is a derivative of "בורר"/sifting or "מרקד" /shaking in the sieve, and the Halakha is according to the Sages.

But the sages say: one may not suspend a strainer on festivals, nor pour [wine] through a suspended [strainer] on Shabbat, but one may pour [it] through a suspended [strainer] on festivals. The sages disagree with Rabbi Eliezer on both issues. Since she could have prepared the strainer before the festival, it is forbidden to do so on the festival itself. This is the same opinion as Rabbi Akiva’s in 19:1 vessels which can be prepared before Shabbat or a festival cannot be prepared on Shabbat or a festival. Secondly, the sages hold that pouring the wine over the strainer is an act similar enough to “selecting” that it is prohibited on Shabbat. This is only permitted on festivals since on festivals food preparation is permitted.