As Long As The Soul Remains

If one hasn't made havdala he may do so until Tuesday. Why Tuesday??

The soul is composed of [at least] three parts, nefesh, ruach and neshama [this is not the forum to explain those terms. See, among many other sefarim, Nefesh Hachaim, who clearly defines the terms]. On Shabbos we receive a special neshama yeseira comprised of all three elements. On Sunday the neshama departs, on Monday the ruach departs and on Tuesday the nefesh departs, so on Tuesday the neshama yeseira is completely gone. This is similar to the halacha that one may bentch until the food has been digested. Similarly one may make havdala as long as the special Shabbos neshama is extant.

[Rav Avrohom of Sochochov based on the Arizal - Neos Deshe vol. 1 page 145]