The month of Elul is meant as a time of self-introspection for the entire Jewish nation. It is therefore customary to pay attention to which Tefillin and Mezuzot need checking and to have them checked at this time.
The Rambam writes: “If one writes Tefillin by himself or buys them from an expert or from any other person and checks them and returns them to their [leather] encasing, he need not check them again even after several years; we are not concerned that a letter among them was erased or punctured. Similarly, Hillel the Elder would say (Talmud Yerushalmi, Eruvin, Chapter 10), ‘These Tefillin are from my mother’s father,’” meaning that the Tefillin that Hillel the Elder donned were the Tefillin of his maternal grandfather. Clearly, according to the Rambam, if one purchases Tefillin which are known to be Kosher, one need not re-check them and one may rely on the assumption that are still Kosher.
The Sefer Orchot Chaim quotes a disagreement among the Tannaim whether one must check his Tefillin occasionally or there is no obligation to check them even after many years, for some say that one must check them every twelve months, meaning once annually. He writes, “I have heard that the French sages customarily check them once a year. I have found in the responsa of the Geonim that if one wears these Tefillin every day, one need not check them even after fifty years since they are open to the air and will not deteriorate from the inside. However, if one wears them only occasionally, they must be checked. Indeed, we have heard from elderly scholars that they must be checked twice every seven years.”
Based on this, our Tefillin which are worn daily need not be checked according to the letter of the law since we need not be concerned that they will deteriorate inside and become invalid. Only a Mezuzah which is constantly affixed on the wall must be checked once every three-and-a-half years. Maran Ha’Shulchan Aruch rules likewise.
Nevertheless, some Poskim write that nowadays when many pairs of Tefillin are found to be invalid because of excessive perspiration which ruins the letters, it is thus proper to check one’s Tefillin twice every seven years as is the Halacha regarding a Mezuzah.
Based on this, Maran Rabbeinu Ovadia Yosef zt”l writes that since nowadays there is an influx of scribes and checkers that are not proficient enough in the laws of Tefillin and Mezuzot and several times invalidations were found in the letters they have written or checked, it is thus worthy to act stringently and have them checked during the month of Elul; regarding Mezuzot, there is even more room for stringency. Nevertheless, according to the letter of the law, one is only obligated to check them once every three-and-a-half years. (Responsa Yechave Da’at, Volume 1, Chapter 49, Yalkut Yosef and Halacha Berura, Chapter 39)
When Maran Rabbeinu Ovadia Yosef zt”l was younger, he and Hagaon Harav Naim Eliyahu zt”l (brother of Hagaon Harav Mordechai Eliyahu zt”l) would sit in one of the Babylonian synagogues in Jerusalem and check many people’s Tefillin. Many of these pairs were found to be absolutely invalid. In this way, they would bring merit to the public by making them aware of the state of their Tefillin so that they would not lose out on this precious Mitzvah.
At this point, we would like to warn the public that when one goes to purchase a Torah scroll, Tefillin, Mezuzot, or a Megillat Esther, one should be exceedingly careful whom one purchases these articles from, for sometimes there are serious halachic issues with their validity and we have personally seen many times that, upon checking Mezuzot and Tefillin, they have proven to be invalid from the onset; this was due to the lack of knowledge on the consumer’s part at the time of purchase. Thus, one should look to purchase such items only from a person who is worthy and upstanding and known for his expertise in this area, for only in this way can an individual be relied upon regarding the validity of Tefillin and Mezuzot.