Layoff notice, what does the 60 days mean? - what worked for you?

I received a formal notice about a mass layoff at my workplace. I do not know if I should start looking now or wait for severance news. Has anyone been through this?

I stayed for the retention bonus and looked for work on the side. It cushioned the gap.

Thanks for writing this out. I went through something close to it 12 months ago, and it got easier once I kept a simple log of every call and form.