Weekend trading

I tried to post new trades at MARKET late Friday but the market closed. C2 gave me a message like: "If you input trades at 17:45 the trades will stay pending until the market opens"…or something like that.

So I set up a whole bunch of trades, hoping that they would activate on Sunday but was very disappointed to find that they had all expired before market open today. I guess I misunderstood and it cost me dearly as I would have had minimally a 2.5% winning day today, instead I was about break-even.

Could you summarize the rules then for holding MARKET orders over the weekend?

Thank you.