The original Matan Torah — meaning, when we celebrate Shavuos — is the time when we mistakenly thereafter served the Golden Calf. Why would we celebrate that day? It is equivalent to celebrating after marrying one's spouse on a different day — the day the couple broke their first engagement.
The Golden Calf was made on the 16th of Tammuz, forty (actually, 39) days after the Giving of the Torah. But that was not yet in evidence on Shavuos, or for the next 38 days afterwards. During that time, rather, we were able to rejoice with the Torah we had received and with the tremendous spiritual benefits we obtained along with it. So, "at a time of rejoicing we rejoice, and at a time of mourning we mourn" (Bereishis Rabbah 27:4).
It is true that in a sense, Moshe's shattering the Tablets on the 17th of Tammuz was like the breakup of a marriage (Shemos Rabbah 46:1). But with the reconciliation between us and Hashem on Yom Kippur, we regained much of what we had lost (plus, as a bonus, the entire Oral Torah - ibid.)
And after all, Shavuos commemorates, among other things:
A unique occurrence in history: that Hashem revealed Himself to an entire nation (Deut. 4:32ff).
The provision of a purpose for all of Creation to continue to exist (Shabbos 88a).
The end of the break between heaven and earth, between the physical and the spiritual realms: "the lower ones may ascend on high, the higher ones may descend below, and I [says Hashem] will be the first to start" (Shemos Rabbah 12:3). As explained in many places in Chassidus, before the Giving of the Torah, no physical object used for a mitzvah could become intrinsically sacred; whereas now you can, for example, take the skin of a cow and turn it into a holy object - a Torah scroll, a pair of tefillin - and it retains that kedushah (even when it is no longer fit for use). This is what makes us capable of turning our world into "a dwelling place for G-d" (Tanchuma, Naso 16), the ultimate purpose of Creation.