I use an ice cream tub, hold it underneath, tip the plant over and tap it. If the seeds are ripe they drop out. Make sure they're completely dry and store them in paper packets.
On a dry day, after the morning dew has gone. eg late afternoon.
It's about when the seeds are ripe, not the month.
For hellebores it's April or May. Some plants haven't finished flowering yet so it will be at least October or maybe November.
Some seeds are best sown fresh, others (agastache included) are best kept dry and cool til spring.