It seems like only yesterday that I was sitting shiva, yet the reality is that shloshim is ending already. I'm feel somewhat uncomfortable with the idea of a seudah and a siyum to commemorate the date -- it just seems too festive, too celebratory. The Rambam describes aveilus like this:
כל שלושה ימים הראשונים, יראה את עצמו כאילו חרב מונחת לו על כתפו; משלושה ועד שבעה, מונחת בקרן זווית; מכאן ואילך, עוברת כנגדו בשוק. כל זה כדי להכין עצמו לחזור, וייעור משינתו, והרי הוא אומר "הכית אותם ולא חלו" (ירמיהו ה,ג), מכלל שצריך להקיץ ולחול
Pretty serious stuff. The Nitei Gavriel (here) quotes a view that even if a siyum is done as a zechus for the niftar, if the aveil did not participate in the learning (let's say the siyum was done right after shiva), the aveil should serve as a waiter (to diminish his simcha) in order to be able to participate in the meal. Yes, that's probably a chumra, but it reflects the mindset of what aveilus is all about. That being said, l'ma'aseh people do mark the date of shloshim with a seudah, just like many, if not most people these days, celebrate a yahrzeit with food and drink, not by fasting. Introspection and reflection are hard to do; having a sip of schnaps and a piece of cake is so much easier and so much more enjoyable.
Our parsha tells us, "Adam ki yakriv...," speaking of an individual who chooses to offer a korban, yet that same pasuk ends with the words, "takrivu es korbanchen," speaking in the plural tense about those who come to offer korbanos. Why the switch from singular to plural? The Ishbitzer in Mei haShiloach explains that when an individual chooses to make a sacrifice, that act has an impact and makes an impression and causes others to be inspired and follow suit.
The purpose of making a siyum, of saying kaddish, is to show that the sacrifices of a parent, the "adam ki yakriv," inspire a "takrivu es korbanchem" -- children, grandchildren, neighbors, friends learning more mishnayos, davening better, being becoming better people.
Ending shiva or ending shloshim is not about "letting go." Aderaba, it's about showing that the niftar continues to be with us and inspire us. The Kozhiglover, in explaining why chassidim celebrate and do not mourn or fast on a yahrzeit, quotes a Midrash on Ya'akov's words, "Ani ne'esaf el ami": Ya'akov told his children that "im z'chisem lachem - z'chisem b'tazmi," but if not, when I leave this world I will just return to my fathers. When we do good, it's "b'atzmi z'chisem" -- the spiritual presnce, the "atzmiyus" of that person, is still there with us, expressing itself though the mitzvos and Torah we engage in.
In giving an accounting of the money and gifts donated to the Mishkan, last week's parsha tells us that there was "dayam v'hoseir," enough plus leftover. The Ohr haChaim famously asks how these two descriptions can simultaneously be true -- if there was enough, it means there wasn't any leftovers; if there was leftovers, then there was more than enough, not "dayam."
In one of the hespeidim given for my father, the speaker compared life to building a Mishkan. The Torah speaks of "v'shachanti b'socham," inside the people, not inside the building -- we are the Mishkan. I would like to suggest an answer to the Ohr haChaim's question that fits that context. An individual is given just enough time in this world to accomplish whatever they are supposed to accomplish -- it's "dayam," no more and no less. But even after the person is gone, there is a "hoseir," there are the leftover memories and thoughts of the person that continue to be with us.