4. Based on the phrase, "and you will honor it by not...speaking of [mundane] things" (Isaiah 58:13), our Sages inferred that: "The way you speak on Shabbos should not resemble the way you speak during the week" (1).
Therefore, it is forbidden to say, "I shall do such and such tomorrow," or "I shall purchase this merchandise tomorrow"; this only applies if the performance of the task being mentioned, is forbidden on Shabbos under all circumstances (2). If, however, it is theoretically possible to accomplish the task on Shabbos [in a permitted manner], it would be permissible to talk about that task, even though the circumstances don't allow for it to be performed at that particular time,
Thus, it is permissible to say, "I shall go to such and such a place tomorrow," as long as one's choice of words does not imply that one will be travelling in a wagon (3). One should not, however, dwell on the subject.
It is prohibited to speak at length, on Shabbos, about matters of no consequence ("sicha beteilah"), and it is forbidden to speak of anything that causes emotional pain. It is also forbidden to make verbal business calculations on Shabbos. This applies both to calculations of future expenses and to the review of past accounts which are still relevant; therefore, it is forbidden to say: "I have spent such and such on workers' wages for this building," if he still has some of the money set aside for workers' wages in his possession, and he thus needs to take account.
Accounts that are of no purpose at all, however, may be calculated, but one should not dwell on them, because it is forbidden to speak at length on matters of no consequence, as mentioned above.
(1) Tractate Shabbos 113a
(2) Even if it is only a rabbinical prohibition. There are those who rule that talking about performing a mitzvah after Shabbos is permitted, even if that mitzvah involves an activity prohibited on Shabbos. However, the Mishna Berura (307;1) rules that one should be stringent (and not even talk about a mitzvah), if it is not essential to discuss it on Shabbos.
(3) This is permitted even if one intends to travel with a wagon.