“I'm expecting something to come in today or tomorrow.”
You're waiting for a package to be delivered to you at work. You need to get it as soon as possible, so you checked with the receptionist. She said that no packages have come for you. You say this to explain why you're asking her.
I'm expecting something to come in today or tomorrow.
"Expecting ___" is used to talk about things that you think are going to happen in the future. Use "expecting ___" when there's something that you will be surprised about if it doesn't happen. For example:
Hey, where's Sammy? I was expecting him to be here.
You can talk about objects or events that you're expecting:
I'm expecting a package.
They're expecting rain tomorrow.
I'm expecting a phone call.
Or you can use the phrase "expecting ___ to ___" to talk about actions that you expect:
I'm expecting them to get back to me with an answer by the end of the week.
"Come in" is a phrase that's used to describe things being delivered to a business or workplace. This can be used for packages, letters, and other deliveries:
A shipment of paper came in this morning.
But it can also refer to money, sales, or information:
The results of our quarterly review came in yesterday.
Over 7 million dollars in revenue has come in since the product launched.
This is a really simple phrase, but just notice that "today or tomorrow" is a better and more natural phrase than "in the next day" or "in the next two days". It's also better than "tomorrow or today".