From the pasuk (verse) (Devarim 16:15) of ve-hayitah akh same'ah – "and you shall be altogether joyful" - we learn that there is a commandment to be joyous on the three major holidays of the year – Pesah, Shavu'ot and Sukkot. One of the Gemara's concerns revolves around the question of how we can joyously celebrate the first night of the holidays, when any korban that had been brought prior to the Yom Tov cannot be considered part of the joy of that holiday, yet there has not yet been an opportunity to sacrifice the korban hagigah for the holiday!
This question is of such concern to the Gemara, that it considers the possibility that the command to be joyous on Yom Tov that appears in the above passage, does not apply to the first night of the holiday; the word akh is understood to possibly limit the obligation to the rest of Yom Tov.
The Gemara's question is based on the assumption that the mitzvah of simha – of joy on the holiday – is defined, at least during Temple times, as partaking of the sacrifices, and eating the meat associated with them. According to this view, other activities that bring joy to a person are simply not included under the specific definition of the word simha as it is used in this context. The conclusion of the Gemara rejects this approach, however, by suggesting that after the destruction of the Temple the joy of Yom Tov can be had by wearing freshly laundered clothing, special colorful clothing for women, by drinking wine, etc. Once the Gemara suggests this view, apparently it accepts the possibility that even while the Temple was standing and fully operational, these activities could legitimately be considered participation in simhat ha-hag – the joyousness of the holiday. Therefore we can conclude that even on the first night of Yom Tov there is an obligation of simha.