How to use a literal curly brace "{" in XAML?

Use {} as the escape sequence.

ConverterParameter = "{}{0}/{1}"

For those who got here looking for the answer for WinRT-XAML. The escape sequence is \ sign. Although this documentation states ' should work but on the other hand this example uses \ (which actually worked for me).

You can use:

ConverterParameter = "{}{0}/{1}"

More information can be found here.