Taanit, Chapter Two, Mishnah Nine
This mishnah discusses on what day the series of fasts can begin.
1) They do not decree upon the community a fast to begin on a Thursday in order not to cause a rise in the market prices.
2) Rather the first three fasts are held [in this order], Monday, Thursday, and Monday; the second three, Thursday, Monday, and Thursday.
a) Rabbi Yose says: just as the first three [fasts] should not begin on a Thursday so too neither the second [three] nor the last [seven].
Section one: There are several explanations as to why the market prices will rise if a series of fasts is decreed on Thursday. One explanation is that if the store owners see people buying big meals on a Thursday, a meal for the end of the fast and large meals for Shabbat, they will think that a famine has struck and they will raise the prices. This is not a problem on Monday which is not next to Shabbat and so people are buying less food.
Another explanation is that fasting so close to Shabbat will cause the storeowners to think that there will be a great panic and they will raise the prices.
Yet another explanation is that since the villagers bring the food to sell in the market on Thursday, and they will not know that there is a fast ahead of time, they will not bring enough food for the Shabbat and the break-fast meal and the great demand will cause a rise in the prices.
What I find interesting is that in times of crisis the rabbis were cautious not to impose additional financial hardships by decreeing fasts.
Section two: According to the first opinion, while the first three fasts do not begin on Thursday, the second set does. Since the second set is not the beginning of the entire series but rather just a continuation of it, they need not fear that starting on Thursday will cause prices to rise.
Rabbi Yose holds that none of the series of fasts begins on Thursday.