The Free Dictionary defines herald as
- A person who carries or proclaims important news; a messenger.
- One that gives a sign or indication of something to come; a harbinger: The crocus is a herald of spring.
Just as the crocus is a herald of spring, the flower can also herald in the season. To herald in something is to announce the beginning of something important. It's often used for holidays, the four seasons, and anything the writer considers important or eventful.
Headline: Fair Final Days of Summer, But Rain to Herald in Fall on Sunday | 20 Sep 2013 ·Right on cue, typical fall rains will usher in the official start of autumn with the equinox on Sunday.
25 Jan 2018 · The first snowdrops appear here in Somerset around the end of January and they are a sign to herald in the end of Winter.
An exposition of the Sacred Tooth Relic of the Lord Buddha will be held from 6 to 16 March 2009, to herald in a new era of peace and unity in Sri Lanka.
19 Oct 2016 · It would be great to see the New Year's Eve fireworks back to herald in 2017.
In the first example, the writer is using herald in and usher in to mean the same thing, but a metaphor from the theater is less colorful than one where anyone who's ever seen a film set in the Middle Ages is going to imagine some young man in red tights with a trumpet, i.e., a herald. Thus one often sees herald in in headlines commanding your attention.