You are right about Ranunculus being finished flowering and Zinnias won't have started to flower by that date. Ranunculus don't like temperatures much above 60 degrees - they are best grown in California or the Pacific northwest during the winter months. Or as an Easter flower grown in greenhouses in the north. Zinnias are at their best in mid to late summer.
If you aren't good at timing flowers, it would be best to leave it to the professionals - I have seen too many times someone that wanted to have flowers for their wedding that they grew themselves and it turned out to be a big waste of time and money. Just being realistic here - you will probably have enough going on to plan your wedding that you shouldn't have to worry about growing the flowers and having them look good or to be ready for a specific date. I have grown flowers and plants my whole life, and when anyone asks me for specific flowers with specific colors on a certain date, I ALWAYS have turned them down. Unless you have a really large garden with lots of flowers to choose from, it would be best to buy them locally. If you know of any farmer's markets in your area, check with the flower vendors a couple of weeks before your date and ask them what will be looking good and what they can provide you with on your wedding day. Trust me, it will avoid headaches.