Towards the end of today’s daf (Yevamot 43b) we find a Beraita listing what may or may not be done in the nine days between Rosh Chodesh Av and Tisha B’Av, and while it states that neither Eirusin nor Nissu’in may be performed during this period, the Gemara then clarifies and explains that though Nissu’in may not be performed, a couple may commit themselves to one another at this time through Eirusin but without accompanying their commitment with a festive meal – which would then transform this into being a שמחה which we avoid during this period.
Significantly, this same distinction applies in terms of the laws of Chol HaMoed where we are taught (Moed Katan 18b) that while Eirusin (accompanied by a festive meal) or Nissu’in (even not accompanied by a festive meal) may not be performed on Chol HaMoed due to the concern of אין מערבין שמחה בשמחה (we do not mix one simcha with one another), Eirusin without a meal may be held during Chol HaMoed (nb. while meals are not allowed, various forms of gathering are permitted – see Teshuvat HaRivash No. 260 as quoted by the Beit Yosef on Orach Chaim 546, as well as Piskei Teshuvot on OC 546 No. 1).
What this might be understood to mean is that when a couple make the extraordinary commitment to marry each other, then unless there is a festive meal celebrating their commitment then there is no שמחה. But how can this be? Surely an Eirusin is an objectively joyous moment and, in many ways the definition of שמחה! Given this, how can we say – as our Gemara does – that אירוסין בלא סעודה מי איכא שמחה (‘Is there really any joy in an Eirusin without a festive meal?’).
To answer this question we must distinguish between two words: Happiness, and Simcha, and this is because, as Rabbi Sacks explains, while happiness is something that someone can feel on their own, simcha is not a private emotion. Instead, it is a social state such that there is no such thing as feeling simcha alone.
Given this distinction, while it is true that when a couple get engaged – whether during the nine days or on Chol HaMoed or on any other day of the year – they feel happy. Yet what our Sages are saying in our daf and elsewhere is that while this happiness (which, especially at the stage of Eirusin, is also mixed with a sense of nervous excitement about what the future will be) comes from the commitment to marry one another, happiness is elevated to simcha when we invite others for a festive meal to celebrate this moment with us, and it is this form of simcha which they felt the need to restrain both during the nine days (given the more mournful nature of that period), and during Chol HaMoed (due to the concern of not mixing one simcha with another).