My dictionary lists both いっさくねん and おととし as readings of 一昨年. Which of these are commonly used? Is there any difference between these two readings?
おととし is far more common in everyday conversations with your friends and colleagues, whereas いっさくねん is used mainly in formal speeches and presentations. いっさくねん is the on-yomi of this word, so it's not surprising that いっさくねん sounds more formal and/or technical than おととし.
However, when people write おととし, they tend to use hiragana (it depends on the writer's taste). This is partly because おととし is a non-straightforward 熟字訓.
So when you see 一昨年 actually written on paper, and have to read it out loud, いっさくねん may be a better choice.