Discuss how, on its surface, the ode "To Autumn" seems to be little more than description, an illustration of a season. But underneath its...

This ode is about a moment in time but it also is about change and transformation. Therefore, time is a significant theme here. Consider the first two lines: 

Season of mists and mellow fruitfulness, 

Close bosom-friend of the maturing sun; 

Autumn is the season when the warm weather is turning cold. This produces condensation and "mist." Autumn is the end of the growing season. It is the season when farmers harvest their crops. Ideally, the harvest is bursting with "fruitfulness." Thus, it is a time when things are ripe and full of life.

Keats is also making a comparison between the progression of seasons and the progression of the day. Spring is morning and rebirth, Summer is the bulk of the day and growth, Autumn is evening and harvest, Winter is night and death. Here, Keats is comparing autumn to the evening. Thus, autumn is a close friend of the "maturing sun" or the sun as it sets at the end of the day. 

Although notions of evening, the end of growth, and the approaching winter might be described in melancholy terms, Keats celebrates autumn for its particular beauty. The first stanza contains imagery of fruits and plants fully ripe and therefore at their peak conditions. Despite the approaching end of things that upcoming winter represents, autumn is a beautiful time and notion in and of itself. 

The second stanza illustrates ideas about the harvest. Keats describes autumn as "the gleaner." That is, one who gathers the grain. Given the suggested notions of evening at maturing age, he also seems to be saying that autumn is a time for gathering the fruits of our life's labor. It is therefore a time to gather, appreciate, and extract from life whatever we can. Perhaps he is suggesting that as we get older, we should appreciate (gather/glean) life especially in these later stages. 

In the final stanza, the speaker (Keats) basically tells Autumn not to worry about the songs of spring. Autumn has its own songs. In other words, this season, the evening, and the mature stage of life all have elements to be celebrated. This poem is about appreciating autumn as a season. But the symbolism provokes the reader to consider the passage of time in order to appreciate the end of the day and the end stages or the penultimate times of life.