When you should put down a pre emergent weed killer?

depends on your location

To determine the best time of year to apply pre-emergents, note when the weeds begin to sprout this year and count back two or three weeks. That's when you should apply a pre-emergent next year.

At least 2-3 weeks before weeds start emerging.

In Feb. before the weeds start growing. If they pop up anyway, keep the grass and weeds cut frequently so the weeds won’t go to seed. Best wishes ☺️

I associate timing with forcythia blossoms.

Heres a link for your consideration, the seasonal weather timing may be important

https://www.oneprojectcloser.com/when-and-how-to-apply-weed-feed/

Once you don't have any more snow and the soil is workable. Remove any that may be already have started. If the ground is still frozen it may just wash away if it rains, so wait until it is ok for at least an inch or so. For the garden, wait until you have tilled, then put it down, again taking out anything that has already emerged.

Here are a couple links to help you out! Ann is correct, a few weeks before weeds start sprouting.

http://www.hgtv.com/design/outdoor-design/landscaping-and-hardscaping/when-to-apply-herbicides

https://www.solutionsstores.com/pre-emergent-herbicides