well, before someone can tell you WHEN to plant them, you need to tell us where your located at, as location matters as to when you plant them
but general rule of thumb, is about few weeks after your LAST frost is expected will work, as long as temps stay above 50 degree's or so
they can be planted in most northern states, say north of interstate 80, from about Mid May till mid to late June,
if in the south, I cannot really say, as you have more worries about heat than cold tempos to deal with!
and since I have no clue as to how much land your planting here,I will add this,
as in most places a LOT of things will eat soybeans , from the time they sprout till time to be harvested, ,so they might/will get hammered fast if you have many deer in your area! and maybe never make it to hunting season, unless you fence it off or other wise protect it!
an acre of beans on my place won';t last more than a few weeks one ready to harvest,,
heck, 3 acres doesn't make it till end of Dec for me!, fully ready to harvest at about end of OCT to first week or so in nov!, so I can loose almost 3 acres on a few weeks, and being deer know there there, 24/7, a LOT of deer get nocturnal here and eat at night when its safer!
Food plots have really taught many deer this side IMO!
and a side note, some info about soybeans
is they are typically an easy crop to grow, they don;t need a ton of things to be happy and many times will even grow just tossed onto softer soil, not even needing to be buried much! if at all!
but when your dealing with 1 yr old seed or older, I would recommend you coat it with some inoculate to make sure it has the best chance of growing, so no work goes wasted, most AG stores will carry it and its cheap and easy to coat seeds with it!
and if round up ready beans, its a GOOD idea to come back and spray after they get going pretty good, will allow what ever nutrients in soil get to beans and NOT weeds!, which will make for much lower yields, and with beans again, you want all the yield you can get, as they will take a beating fast once things find them!
last, if you plan to plant more in the yrs to come, if you join The National Turkey federation, they run seed programs and they sell left over 1 yr old seeds, almost dirt cheap, you only get what they have, and its on a first come first serve deal, but its a great way to get a LOT of seed for cheap if you do your part and work with your area's chapter's seed person!
I have bought many 300-400 dollar bags of seeds for 20-25 bucks a bag thru there program, on MANY seeds, NOT just soybeans!