Unfortunately there is no 'set' answer.
Traditionally a Friday & Saturday in the 'High Season' { May to October } was considered the most expensive time to get married with venues often offering mid week and 'out of season' rates to couples who chose to marry Nov - Apr { excluding Dec }.
But now weddings happen all year round and while more do happen during the 'summer' months there is no longer such a difference between a high and low season.
Another major 'cost factor' to consider are flowers. Flowers do get dearer in February & March for Valentine's Day and Mother's Day. This is because growers deliberately grow less then there will be demand for in the run up to Valentine's Day and Mother's Day so they will get a better price for them at auction. While their main focus here is Valentine's Day and Mother's Day, this of course has a knock on effect to any floral related occasion that will happen during these months. { Most flowers are bought wholesale, through auctions, in the likes of Brussels and shipped around the world to flower shops or other wholesale units }
Christmas and News Years brings with it its own extra staffing and overtime costs, so you may find that across all suppliers extra 'seasonal charges' and expenses are incurred. And again some blooms can get a little pricer here too, as can cakes unless you've ordered well in advance!
So I suppose if you were to take all the above into consideration marrying in January, April and November are probably the 'cheapest' months with a mid week wedding giving you the most negotiating power. Therefore making December, February & March and May to October, on a weekend, collectively the most expensive time of year to get married.
{ Of course local, regular, ad hoc, sporting and religious events in your area or country can also have an impact on your wedding costs; any demands placed on resources generally results in a price hike! }
If I had to chose just one single day that was the most expensive to get married on? { Well that is a toughie } But I would imagine, in the majority of areas, its possibly New Years Eve.