House Stark made a deal with House frey to have more men as walder frey told catelyn and ned he'd give them more men if they married robb to his daughter Guest at first catelyn told ned no because she wasn't going to marry her first son off like some milkmaiden

Ned reminded catelyn they needed men if they needed to fight the lannisters so catelyn finally agreed as walder frey told them he was sending his daughter in a month to be married to robb and if robb cuts the deal he'll kill them

Ned tells robb a month before so he can take it all in before it happens so robb doesn't think his family sent some random girl to be wedded to him, The carriage arrives a month later as Guest steps out