So basically, if you have more seniority and want that gig, grieve it. It'll probably all boil down to established past practices in your center, though.
So at my center, most of the 22.4s are kept on the same route most Saturdays. There's the occasional staffing issue that might flip things around but it's been pretty consistent this year. Lately they've been letting RPCD volunteer for Saturday work so that all of us 22.4s aren't out until midnight. When an RPCD would come in, they'd normally be assigned to their M-Fri bid route.
That brought on a little bit of outrage from a few of us because Saturdays are pretty much the only workday we can look forward to any kind of consistency, and it was pointed out that it seemed wrong that an unscheduled driver could bump a scheduled one off of their typical Saturday route just because someone wanted a 6th punch.
Amazingly, management heard us (or maybe our business agent.) RPCDs seem to be filling the gaps on Saturdays now, and 22.4s running their normal Saturday routes. The biggest thing here, though, is none of us picked our Saturday route. We were just thrown on whatever in the beginning and so long as there wasn't any kind of massive screw-ups then you were kept where they first put you.