So in the campaign I DM there is a ritual that is supposed to take ⅔ of a year. Obviously, we need to skip most of these days, and some of the events deserve only a bit of role playing and that's it.

On the other hand, my players love to go into details, and it causes lags. For example they solved the mystery of a curse, got everything they needed to break it, and then it resulted in round by round PvP that soon got boring for everyone. I could have told them that this was a part of the story that isn't important. That identifying what happened and gathering ingredients was the interesting part. But I'd need to break immersion to do this.

Using my own advice, I plan to have another session zero and another grab on the Same Page Tool, but I'd like to have some ideas first, and I don't want to reinvent the wheel.

So is there any tried way to communicate to players that

this isn't the important part, don't use too much time on it, more interesting things are waiting to happen!

without breaking immersion too much?

I'm interested in saving real play time, not in-game one. Players are aware of time constrains, and in-game they are doing great to progress as fast as feasible.

The easiest way I can think of, for players to accept a time skip is to ask them how are they planning to invest the following months.

This is something easy to land because that investment is going to give them some kind of benefit. It's a little bit like playing Santa Claus, players tell you what are they going to do, you give them something they got because they were working hard on it and not going on wacky filler adventures.

You don't need to give them magic items or bonuses, it might be a development in their character plot, some juicy info, information about future dangers... That way nobody has the feeling they just wasted that in-game time and you can jump right into the important stuff.

I could have told them that this was a part of the story that isn't important. That identifying what happened and gathering ingredients was the interesting part.

As a DM I've run into this situation before. You can always try to find an indirect way of hinting to the players that what they are doing doesn't have to be done. In my case the players were in a dungeon where they found some weird ingredients which weren't meant to do anything (they were there for decoration), but the players were convinced that they can unlock something with them and kept trying different things. After a while I said:

It's getting dark, and it gets harder and more dangerous to travel around

At this point everyone got the hint that there's not much else to do in the dungeon, and they quickly returned to their base.

In your case, if you want 2/3 of a year to go by fast, you could always do some narration like:

And like the days went by, you were training to prepare yourselfs, and the day was finally arriving for the ritual to be completed.

To be honest though, I would try to avoid skipping long periods of time like that, by making the ritual shorter, or by introducing mini-quests in-between. But that's only possible if your campaign allows it.

So is there any tried way to communicate to players that

this isn't the important part, don't use too much time on it, more interesting things are waiting to happen!

It's probably not the best idea to tell players that "interesting things are waiting to happen!". It's probably better to attract their attention with something in-game that would capture their interest. Maybe a dragon is flying to the general direction you want the players to go. Maybe an explosion happened in that area. Some screams of agony are echoing towards it. An NPC got attacked around there. Find the best that can be attached to your campaign.

Just Tell Them

Some gaming groups are lucky enough to have everyone on the same wavelength (whether by Session Zero, personal perceptivity, or just plain good luck) and can manage these things without saying it directly.

That's... relatively rare, in my experience. Not unheard of, but relatively rare.

What happens far more often in my experience is that if the GM wants to zoom out to a longer time scale, he or she will just say it: "We're moving to a longer time scale." Or the player will phrase it as a request, either publicly or privately, but still unambiguously. It's so much more effective than hinting and hoping.

Your main concern in this example (the eight month ritual example) seems to be breaking immersion. My plain but multiply reinforced experience is that saying "Let's zoom out" does not break immersion any more than the act of zooming out. Whatever method you use to get that "zooming out" to happen-- oblique hints, hoping for spontaneous consensus, hoping for spontaneous combustion, telling them to move it along, asking what they're doing two weeks from the present, whatever-- is going to result in a change of mindset from the immediate minutiae to the longer term.

If it's gotta be something, it may as well be an effective something.

Try to time it for an end of session.

That way, you can tell the people as they are putting away their books and dice, "This ritual will take X time. Write out what you plan on doing during that time in either individual or group."

This lets you use out of play down time without taking time away from game play.

If you can get them to email that to you a few days in advance (congratulations, then!!!), you can figure out how to resolve their down time for the next session. You may end up with a couple of side adventures before the main plot gets back on track.