What should I do if friends are going out without bringing me?

I usually go out with friends every week but they didn't inform me of where we gonna go this week. I ume they forgot to invite me or just wanted me out this time. It always makes me feel saddened if they decided not to bring me along. I tried to get in touch with them but it seems everyone is busy working at this hour. I hope they forgot to invite me so once I get hold of them I can still request to join them. But if they intentionally left me out due to whatever reason, it won't make me happy and it's the end of the world. So what should I do in order to prevent major depression if my friends did left me out this week?