When I tie my shoes, I generally do not open them for a while (a month or so). I slip my feet out. Am I permitted to tie my shoe on Shabbos, since most people do not do this?
Although the type of knot that you tie is not generally intended for a long time, it is not permitted to tie a knot on Shabbos (even a weak knot) with the intention of leaving it tied for a long time, and certainly if the intention is to leave it for more than seven days.
However, this prohibition applies only when there is definite intention to leave the knot for a lengthy period of time.
The opinion of the Kol Bo and Hagahos Maimonios, as quoted in the Rema 317:1, is that any knot that is tied fore more than 24 hours is considered semi-permanent, and it is forbidden to tie it (rabbinic prohibition) according to Rashi. According to the Tur and the Mordechai, a knot that is tied for seven days or more is called semi-permanent.
The Biur Halacha (317:4) writes that we should be concerned for the opinion of the Kol Bo, and allows leniency only under extenuating circumstances.
Although the principle halachah follows the Rambam, who maintains that one’s intention is not sufficient to create a prohibition, the custom is to be concerned for Rashi. Therefore, even a knot that is not well-crafted and firm cannot be tied in order to leave it for 24 hours or more unless there is a pressing need. Even in case of pressing need, one may not tie any knot with the intention of leaving it for more than a week.