rainwater calculation

Yes, it should be calculated in July. July is when there will be peak demand for irrigation so you need to show that you will be able to capture enough rainwater to meet that demand.

Scott, Ebru, I have to disagree, or at least, it depends on the location of the project. In the LEED Reference Guide with Alternative Compliance Paths for Projects outside the US, you can find the following statement:
6. Calculations
See the LEED 2009 Green Building Design and Construction Reference Guide for calculations regarding this credit. The following should replace the first bullet under Standard Assumptions and Variables:
 All calculations are based on irrigation during the month with the highest irrigation demand.
The evapotranspiration rate (ETo) for the month with the greatest irrigation demand should be determined by the project’s landscape designer based on local climate data; project teams may also refer to the International Water Management Institute (http://www.iwmi.cgiar.org/WAtlas/Default.aspx) or the EPA Water Budget Data Finder in determining the peak watering month with the greatest irrigation demand (http://www.epa.gov/WaterSense/new_homes/wb_data_finder.html).

I'm located in Mexico, so this approach has been useful for project documentation here. Let's not forget that every region has its particular features.

Cheers!

Yes, I agree with Nadia. I only really gave have an answer without considering regional needs. The month of June is less important than determining the actual month of peak irrigation demand. This will most likely not be during annual peak but during the dry season when captured rainwater availability will be at its lowest.