During the winter of 1840-41 John Bidwell, who had lost his land to a claim jumper, taught school in Missouri and got ready to move on. The tales of fur trader Antoine Robidoux and the letters of John Marsh extolling the beauties of California generated a great deal of interest among the Missourians. During that winter Bidwell and others organized the Western Emigration Society.
“A pledge was drawn up in which every signer agreed to purchase a suitable outfit, and to rendezvous at Sapling Grove in what is now the state of Kansas, on the 9th of the following May, armed and equipped to cross the Rocky Mountains to California. We called ourselves the Western Emigration Society, ans as soon as the pledge was drawn up every one who agreed to come signed his name to it, and it took like wildfire.” (Bidwell Dictation 1877.)
Why wait until May to travel? For the simple reason that no progress could be made until the grass was grown up enough to support horses and oxen. Travel could not begin while there was still snow on the ground and the animals had nothing to eat. This left a window of about 6 months for travel, which might seem like plenty, but any delay in starting or holdup along the way could jeopardize the lives of the pioneers. An early start and steady progress were essential if the pioneers wanted to reach their goal safely.