Holly-lily

People who go to church only on Christmas and Easter sometimes are called "C and E" or "holly-lily" Christians, or even "Easter bunnies."

Source: wiktionary

In church circles, when pastors and staff and church members plan for the Holly-Lily influx (printing extra bulletins, for example, or preparing the overflow room off of the sanctuary), the feeling is usually less than affectionate. To congregants who support the church Sunday after Sunday, Holly-Lily folks can be easily stereotyped as slackers who want a good church without the hassle of participation. To pastors who are exhausted from extra holiday sermons and services, not to mention the stress of preaching for folks who hear only two sermons a year, it can be tempting to see the Holly-Lily crowd's financial gifts as the only blessing of their attendance.

Source: wiktionary

I've always heard of them as the holly-lily crowd.

Source: wiktionary

Ooh I’ve never heard Christers before! We call them Holly-Lily Christians where I’m from

Source: wiktionary