New research shows that there can be negative effects from last minute revision or "cramming" as it is sometimes called.
If you have revised 75% of the information you need to know for a subject test/exam and then, at the last minute, try to revise the remaining 25%, this last minute "cramming" may erase quite a lot of what you had earlier learnt. Just like all life on earth, memories appear to compete for resources and space.
So, if you can plan your revision so that you have revised all you need to know before the test/exam you should be better able to retrieve this information (memory data) from the long time memory base in your brain.
Here comes the link to the research: