June 20 is a very good time to perform a Mercury ritual. While it is true there is a waning Moon, I consider this to be a less important requirement (and leave it to the so called experts to pass judgement). What is important in my opinion is that Mercury's condition at that time is kinda rare to find, as I will detail further on.
Since I wrote Los cielos se abren and detailed on general astrological requirements in there, the following chart should be helpful for people in South America tune their rituals to the appropriate time. And you can benefit from this election date even if you're not in South America by moving the chart to have Mercury on the ASC or MC.
The chart is set for Santiago, Chile:






