For the last 8 years or so, ~April 20th has been very safe. I usually pick the first weekend after ~April 15th, which doesn't have an overnight low under 38 on the 10 day forecast. Haven't had an issue yet.
Thankfully some of the stores are starting to break the "rule", because for a long time it was impossible to even get plants before Mother's Day.
I'm guessing that the lawn itself is incidental, and by not mowing you're letting flowers (including dandelions and clover?) grow?
Also, I'm gonna be up front about my ulterior motive: I hardly mow my lawn anyways and would love to have another reason to justify / rationalize what I'm already doing :)
But still - genuine question.
There's debate about how effective it is, but that's the theory at least.
The perfect lawn is a relatively new thing - blame Scott lawn and other lawn-care companies.
This year, this year the day before Mother's Day saw a hailstorm that decimated much of our cold-weather early crops (arugula, peas, spinach etc.)