if you choose to do a mission thats needs to be done asap :no
but there are some missions that require you to do them on a certain day.
all the time anytime there is free roam. free roam was confirmed ages ago.
Everything that happens in single player you can do on free roam. everybody can have a 6 star wanted level and all be chased by police. or you can watch police chase other people.
GTA IV HAS NICE A$$ CARS!!!!!