A la Saint Glinglin
Saint Glinglin is a fictional French saint. The name probably represents humorous onomatopoeia. The exact origin is unknown, but there are various fanciful theories. The expression dates back to the year 1897.
The French expression À la Saint Glinglin (on Saint Glinglin's Day) means "some day in the indefinite future, or quite likely never".
Similar concepts in other languages are the German Sankt Nimmerlein, the Portuguese São Nunca (Saint Never), and the Dutch Sint Juttemis.
I believe this phrase will come in quite handy when I explain to my children when we will be getting a dog.