The time of our happiness as it is called in the liturgy. Of the three pilgrimage festivals, Pesach and Shavuot commemorate specific events, the Exodus from Egypt and the receiving of the Torah, respectively. All three have agricultural ties, but Sukkkot, which starts in a few hours does not commemorate a specific event in Jewish history. Yes, it represents the sukkot that people lived during the harvest, and perhaps those that the Israelites inhabited during the 40 year journey across the desert. Also, we rejoice over having made it through Yom Kippur, and increase our joy with the simchat bet hashoeva (water drawing ceremony), expressing the belief that God will bring the appropriate amount of rain in the upcoming rainy season.
The Ashkenazi custom is to omit the recitation of the psalms and the poem Lcha Dodi when Shabbat and a festival coincide. Sepharadi customs vary. The reason given is that we are already in a semi-holiday (even during the intermediate days of Passover and Sukkot). Passover, which celebrates freedom, is not dedicated to happiness in the same way as Sukkot. Perhaps you could make an argument for reciting the psalms of praise during Passover, while omitting them during Sukkot. On the other hand, perhaps you would say them on Sukkot to continue to increase the joy, while omitting them on Passover when we recall, as part of the Passover Seder, that people suffered at the expense of our freedom. Just a thought.
The Torah tells us in Deut. 16:14-15 that we should rejoice and have nothing but joy.
I hope we all have the opportunity to enjoy the time of our happiness to its fullest, as well as the most meaningful and respectful manner. I am gong to attempt to write about on the psalms of Hallel in the coming days, so be on the look out for that. There are also some pictures available on Shibbles' Eyes.