How is summer intemperate in "Shall I compare thee to a summers day?"
If something is "temperate" that means that it is not too extreme. In the case of weather, not too hot, not too cold, not too windy, and so on. In this poem, Shakespeare is saying that a summer's day is not temperate enough -- it is too extreme -- to be compared to his love.
Specifically, he says that summer days can be too hot (the eye of heaven shines too hot). And he says that they can be too windy (rough winds shake "the darling buds of May").
For those reasons, they are not a good comparison for how wonderful his love is.
About this sonnet there is little that one can say except that it is one of the foremost poems in the language. The last two lines climax the poem’s argument. Here the speaker asserts that the poem will endure as long as human life endures, and that as long as people can see (and read), the sonnet will give life to the woman the speaker is addressing. It is the immortality of art that the speaker exalts over life’s transience.
The word temperate (line 2) operates on both a literal and the figurative level. On the literal level, temperate refers to a fact of weather and climate, neither too hot nor too cold for comfort. On the figurative level, it refers to a fact of personality and a state of emotional calm, neither too passionate nor too apathetic for comfort. The word, equally applicable in two directions, thus creates a perfect meeting ground for the weather and the woman.