I'm not really sure what holidays Whoville has other than Christmas. Whoville is located on a Snowflake an/or a dandelion puff but I am not sure what country those items are located in. Somewhere that has elephants and snow. Elephants typically don't live in cold climates but are allowed to play in the snow at zoos, thought this is the cause of some controversy.(1)
I guess if they have Christmas, they're Christian... but I don't think they're Catholic. The Who's secular treatment of the holiday season screams non practicing Protestant to me. But it doesn't matter because there aren't any big summer Christian religious holidays and I WILL NOT settle for a Springtime movie where the Grinch steals Easter. I would settle for a fall Grinch, even though we technically already got that from "It's Grinch Night" aka "Halloween is Grinch Night".
Perhaps the tables could be turned and we find out the Grinch wants to observe the Pagan Summer Solstice by collecting herbs and greeting the sunrise(2) and the Whos steal his holiday from him by attempting to commercialize it. The Grinch is appalled by their saccharine Summer Solstice cards that cost $4 at the CVS. Cindy Lou, the perpetual voice of reason, would then have to shame the Whos as she always does with her wisdom that is well beyond her young years.
Or the Grinch could just have a vacation because he deserves it. Picture this: the Grinch steals a spa day. He could sneakily peel the cucumber slices off of the eyes of another guest to the spa and lay them over his own eyes. The script writes itself.
I do think Grinch would hate fireworks though, so 4th of July is a solid concept.
(1)
https://www.seattletimes.com/nation-world/keeping-elephants-in-cold-is-subject-of-heated-debate/(2)
http://www.patheos.com/blogs/johnbeckett/2018/06/8-things-to-do-for-summer-solstice-as-a-solitary-pagan.html