As beautiful as Shabbat is, it was not God’s intention that humankind live in a constant state of Shabbat. Indeed, it has been understood that because the Torah says, “Six days you shall work and on the seventh day you shall rest,” that it is actually a mitzvah to do creative work on the non-Sabbath days. Additionally, there are numerous mitzvot which one may not perform on the Sabbath.
Shabbat officially ends at the time of the appearance of three medium-sized stars in the sky, but only completely concludes (spiritually) with the recitation of Havdallah.
“...The sons of Rabbi Hisda said to Rabbi Ashi: Amemar once visited our town: lacking wine, we brought him beer, but he would not recite havdalah [over it], ‘and passed the night fasting.’ The next day we took trouble to procure wine for him, whereupon he recited havdalah and ate something...This proves three things;  even one who recites havdalah in the evening service must recite havdalah over a cup;  a person must not eat until he has recited havdalah; and  he who did not recite havdalah at the termination of the Sabbath proceeds to recite havdalah any time during the week” (Pesachim 107a).
The time between the recitation of the evening service* at the end of Shabbat and the recitation of havdallah is therefore an intermediate time when one may perform m’lacha (creative work) but may not eat. If one cannot perform havdalah on Saturday night, they may recite it the next day (as some do in the time zones where Shabbat ends exceptionally late) or even several days later (through Tuesday), although this is not considered ideal.
*One who does not attend or recite maariv, the evening service, may simply recite“Baruch ha’mavdeel bein kodesh l’chol,” "Blessed is he who separates between the holy and the mundane” after the time when three stars would appear in the sky. M’lacha (creative labor) is then permitted but food may not be eaten.
This Treat was last posted on October 8, 2010.
NOTE: As with all Treats dealing with halacha (points of Jewish law), one should consult one's local rabbi for practical application.