What does but thy eternal summer shall not fade mean?

Answer 1

In Shakespeare's England, a year was taken to commence around the 25th of March. Its four seasons started with Spring (a period of birth and growth), followed by Summer (a period of warmth, mature splendor and vitality), Autumn (transition, decline and a yielding up of Summer's riches) and Winter (coldness, sparsity and death).

It was also then common to compare the stages of a person's life to the above four seasons. When Shakespeare said "thy eternal summer shall not fade" he was saying that the glory of his subject's summer - that time when he was at the peak of his powers and attractiveness - would never decline. In his Sonnet 18 he goes on to explain that that this described glory would be preserved through the sonnet living on in the minds of men - far beyond the deaths of both poet and subject.

Answer 2

This phrase is from William Shakespeare's Sonnet 18, where the poet is telling the subject that even though they will eventually grow old and pass away, the beauty of their youth will be immortalized in the lines of the poem. It suggests that the subject's beauty will be preserved forever in the poet's words.