Fasting causes distress and discomfort. This is advantageous for a very good reason - it will inspire us to repent and turn in tefila to Hashem.
When we fast we actually burn away our fat. This is considered no less than burning the fat of korbanos on the mizbeach.
This year Asara b’Teves falls on Erev Shabbos and we will begin Shabbos in a state of fasting. In fact, if Asara b’Teves would fall on Shabbos we would fast, which is not the case with Tisha b’Av. Why does Asara b’Teves carry this stringency?
The Chasam Sofer (Drush l’Zayin Adar) offers the following explanation. If a parent’s yartzeit falls on Shabbos he may not fast as Shabbos is a time of oneg, not inuy. If however he had a bad dream which he would like to annul he may fast even on Shabbos. This is because he is fasting in order to repent and gain a kapara, thereby annulling the gezeira. This will afford him pleasure and is therefore considered oneg.
Most fast days commemorate something tragic. We thus do not observe them on Friday or Shabbos, for Shabbos is not a time for sadness. But Asara b’Teves is different. We do not just commemorate a siege on Yerushalayim for this alone is not a reason to fast (Yerushalayim had many sieges on other days of the year and we do not fast on these days). Rather, it is because from this day it was being decided as to whether the churban would take place, and from that moment until the churban they had the opportunity to do teshuvah.
Chazal tell us that in every dor in which the Beis Hamikdash is not built it is considered as though it was destroyed. Once again this year if, chas v’shalom the Beis Hamikdash is not rebuilt, we will carry the responsibility for its destruction. This is what we commemorate by fasting on Asara b’Teves, the opportunity to avert the churban by doing teshuvah. Just as Klal Yisrael had the opportunity on this day to avert the churban, we too, on this very same day can avert our churban of the Beis Hamikdash not being rebuilt, by fasting and davening.
This is why we even fast on Friday or Shabbos. Asara b’Teves it is not a day of pain like Tisha b'Av, but of an oneg as we realize that repenting affords us a kaparah and the merit to see the speedy rebuilding of the Beis Hamikdash.
Let us use this time to return to Hashem, repent, strengthen our deep connection with Him and thereby bring about the ultimate Geulah.
