The Halacha prohibits talking on Shabbat about doing activities that are a violation of Shabbat. For example, it is prohibited to say, "Tomorrow, I am flying to Miami for a vacation." The Aharonim discuss whether it is permitted to talk about forbidden activity which is a Misva. For example, may one say, "Tomorrow I am going to write a Sefer Torah," or "Tomorrow I am going to circumcise a child" or "Tomorrow I am going to fly to Los Angeles to attend a wedding"? The Magen Abraham (Rabbi Abraham Gombiner, Poland, 1637-1682) rules that talk regarding a Misva is no exception and is prohibited. The Elyah Rabah, as well as the Birkeh Yosef (The Hid"a-Rav Haim Yosef David Azulai, 1724-1806) disagree and are lenient.
The Aruch Hashulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) concurs with the lenient position and reasons that just as "Hefse Shamayim" are permitted, i.e. it is permitted to walk to the city limit on Shabbat for the sake of doing a Misva on Mosa’eh Shabbat, so too it is permitted to talk for the sake of Misva. He argues, why should doing an action to prepare for a Misva be more lenient than talk about a Misva. Rav Shlomo Zalman Aeurbach (Israel, 1910-1995) , as cited by Shemirat Shabbat K'hilhata, refutes that analogy by distinguishing between walking to the city limit which is already accomplishing part of the Misva and talking about performing the Misva which does not accomplish anything. The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) solves this problem by suggesting a compromise. He permits talking about performing a Misva if it will motivate him to be diligent in performing it. Hacham Ovadia concurs with the Mishna Berura's position.
SUMMARY
It is prohibited to talk on Shabbat about performing activities which are not allowed on Shabbat. This includes even activities which are a Misva, unless talking about the Misva will motivate him to actually perform the Misva.
