If you hear that something is supposedly going to happen, it may or may not occur. If a friend is supposedly having a pool party next week, wait until you know for sure before you buy that new swimsuit.
When you add supposedly to a sentence, it means you aren't sure. If a teacher says there is supposedly an early dismissal next week, it means that decision isn't final yet, but it could happen. If a singer is supposedly dating a certain actor, it isn't official — it might just be gossip. Don't confuse supposedly with similar-sounding supposed to be, which can imply a judgment.