Any time a poem asserts that "we were very merry" at the start of each stanza, we're inclined to disbelieve it. After all, who stays "merry" for that long? In the case of "Recuerdo," though, the speaker is so caught up in the joy of re-living a special night that we're inclined to believe her. Whether or not the actual experience of every minute of that night was merry is debatable – but every single moment of her memory definitely is.
The merriness that this poem describes is actually a retrospective happiness. The events of the night aren't as interesting as the speaker's memory of them later on.
The sentiments of one night are so potent that the speaker can capture them even after time, suggesting that they were strong emotions to begin with.