In Harry Potter and the Prisoner of Azkaban, the Weasleys, Harry, and Hermione were staying in the Leaky Cauldron the day before they boarded Hogwarts Express (1st September).
Still in the station, Mrs. Weasley handed out their sandwiches for them from out of her handbag.
"I've made you all sandwiches. Here you are, Ron... no, they're not corned beef... Fred? Where's Fred? Here you are, dear..."
I wonder where (and even when) did she manage to make them all sandwiches for their journey?
She might have made them the day before journey, early morning at the Burrow.
She might have requested Tom the innkeeper at the Leaky Cauldron let her use the kitchen for some time.