Nice attempt, but no go. The term "Easter worshipper" is unprecedented, and doesn't appear in either of your references.
Or are you claiming the issue is the use of "Easter" as an adjective? If that's your "bail-out", then duh - "Easter Sunday", and no need for your inane deflection. Keep trying, it is funny!
I noted the grammatical function when the question was first asked. The above was a reprise.
Your premise was that that usage had not been done before. It has. Three hundred years ago.
Again, ever heard of for example Christmas cactus? Named for the season when it blossoms, and named the same way in French, German and Spanish for the same reason (and sometimes in the US called "Thanksgiving cactus" when horticulturalists vend it early), while in the southern hemisphere in Brazil it's called the "May flower". Plus it sounds nicer than ""Schlumbergera" to the non-Teutonic ear.
Again, a time marker. Adjective qualifying "when". Christmas cactus don't climb down no chimneys and Christmas Island is not where Santa goes for vacation.
Same with "Easter Sunday" for that matter.
Which Sunday? Easter Sunday. Specific. You can denote anything to be scheduled to happen on "Easter Sunday" without it being worship.
At the same time "Easter worshiper" is a bit clumsy as its first implication is that somebody is worshiping Easter.
Easter Parade...
Fourth of July picnic....
Halowe'en candy....
One other aspect that hasn't been mentioned -- the images in the OP are of Twits, which limit their space to (I think) 140 characters. That means things have to be abbreviated. "Easter worshiper" denotes both the time and what the people were doing. It's more specific than "churchgoers" and shorter than "people who went to church for Easter". It sets both the time and the action, ergo developing sympathy. That such worshipers would be "Christian" is already a given; the phrase identifies what they were specifically
doing -- meaning a high level of vulnerability.