Typically deadlines for summer will be from March to May. You might also check to see if they will allow you to start in summer. Many programs do not.
Occassionally, some deadlines are much earlier.
It's probably a really good first step to identify those deadlines so you can plan backwards when to take your GRE and when to start prepping. In fact I suggest that you print this page out and circle this as Step 1 to do this week.
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