While writing in first person's pov, is it always necessary to describe each and every event sequentially? I mean, is it wrong to skip events to a later time and then describe the previous events? Thanks.
Is it wrong? No. Writers do this all the time. Lots of stories have flashbacks, or quick statements like "I suddenly remembered that yesterday Fred had told me that ..."
But be careful not to confuse the reader. If readers start wondering, "Wait, is he talking about something that is happening now or something that happened yesterday?" or "Umm, did his girlfriend break up with him before or after he was in the accident?", well, they're going to get frustrated with your story.
And just the other day I saw a TV show with a totally pointless flashback: It starts out with a scene of a train travelling across the countryside as the credits roll. Then it cuts to a woman who is riding on the train. Then they throw up a subtitle reading "Two months earlier ...". Umm, two months earlier than what? Than this woman that we know nothing about went we know not where on a train?
My point being: I understand starting out with an exciting scene to grab the reader's attention, and then having a flashback to fill in the details. Or having a flashback to reveal information that the characters in the story don't know about when it happens, and only discover later, so that if revealed in sequence it would ruin the mystery or suspense. But flashbacks can be confusing, leaving the reader unsure of what happened when. So the technique should be used cautiously.
In literary fiction, everything is allowed. Read James Joyce, for example.
In more mainstream fiction, novels either follow the chronology of events -- or the narrators train of thoughts. And the latter are usually not chronological, because the szream of consciousness of a fictional character will jump around in time following associations, just like your own thoughts do. But the narration must not appear random, but follow the logic of thought.
In genre novels, chronology is usually a must.