preposition, time

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1. During recess (time), I went to the library.
2. At recess (time), I went to the library.

Gramatically, they are both correct, but do not mean the same thing.

#1 means the for the entire time of recess, you were in the library.

#2 means that when recess came, you went to the library.

(time) is not needed in either sentence.

Thanks, Sullyfish.

"Recess" is an Americanism. BrE speakers would be more likely to say "break".