What traditional festival is written in the poem "Thinking about Chang 'an Hometown on March 9th"? What can be seen from the poem?

Cen Can, a poet in the Tang Dynasty, wrote the poem "Hometown of Chang 'an, Jiu Si in March", which can be seen from the title of the poem "Hometown of Chang 'an, Jiu Si in March", because "March 9" refers to September 9. The whole poem is as follows:

Thinking about Chang 'an's hometown on March 9

On the ninth festival of the ninth day of September, I reluctantly climbed to the heights to look out, but no one could deliver wine in the marching war.

I look at my hometown Chang 'an with a heavy heart. The chrysanthemums around this battlefield are scattered.

The translation is as follows:

On September 9th, the Double Ninth Festival, I finally climbed a high place overlooking it. However, in this war-torn March, no one can deliver wine.

I look at my hometown Chang 'an with a heavy heart, and chrysanthemums are probably blooming sporadically next to this battlefield.