How did the expression try is a come about?

The expression "try is a come" is not a standard English expression. It's possible that you've misheard or misinterpreted a similar phrase.

Here are some possibilities:

* "Try and come": This phrase is used to encourage someone to make an effort to attend or participate in something.

* "Try to come": This is the most common and grammatically correct way to express the idea of making an attempt to attend something.

* "It's a go": This expression indicates that something is confirmed and will happen.

It's important to consider the context in which you heard the phrase to understand its meaning accurately. If you can provide more information about the context, I may be able to help you determine the correct expression.