Totally up to you! We'll be having welcome drinks on the evening of the 5th, but if you don't make it until the morning of the 7th, that works great as well!
Formal Attire is preferred but not required. The reception is in Llano Grande, about 45 minutes outside the city and at a much higher elevation than Medellin, so it can get quite cool at night, as low as 55F. We recommend you bring a sweater or jacket for the evening!