Canada is the Monarch's home for the summer and they stay here until August, when they start migrating south. They pass through United States and stop to take breaks once they get tired. They will typically stop where ever there is milkweed. A group of them stay in Texas before entering Mexico.
The Monarchs fly through the United States.
Milkweed is what butterflies need to see in order to stop and be able to rest thier wings. Without they die from exhaustion.