our 16 yr old asked why we always send her to grandmas the same saturdays. what are other parents actually saying

kid stuff is logistics only, asking how other families have handled this question. so. we have 2 kids, 16 and 13. they go to my moms house roughly every 6 weeks for an overnight or a weekend. always a saturday. and our 16 yr old, who got smart this yr, asked at dinner last tuesday "why is it always saturday and never friday or sunday? do you guys have a thing on saturdays?" we looked at each other. f45 he 47, married 19 yrs, lifestyle since 2019. we send the kids to my mom (my mom doesnt know either, she just thinks we have "couple time" which is technically true) on the saturdays we host or are hosted by a couple, every 6 weeks. its a pattern and shes spotted it. i did the deflection. "we just like to have date nights and grandma loves having you guys, you know that". she did the teen face. you know the face. the "im 16 not stupid" face. she didnt push but she absolutely filed it. now. we are not telling her. obviously. she is 16. that is not info for her. but the question is what is the SUSTAINABLE long term cover story bc shes going to keep asking more questions and shes going to start cross referencing with her brother and theyre going to figure out that "date night" doesnt explain a weekend at grandmas. teens are not stupid and they especially notice patterns when they involve being moved around. the things i have considered as cover stories... we have a "couple of friends we host for dinner whose schedules align with saturdays" (sort of true and harmless, but breaks down if she ever sees the house or hears voices). we have "an adults only book club" (lol but actually some couples i know use this). we have "your dad and i are working on a project we cant explain rn" (creates more questions). we have "we just like having the house to ourselves" (closest to true but invites the "why grandmas specifically" follow up). oh and the bigger thing under this whole thing... at what point do u flip from "manage the cover story" to "the kid is now an adult and either it never comes out at all or it comes out on her terms". like is there a year. is there a moment. when she goes to college? when she gets engaged herself? when shes 30? these arent rhetorical i actually want to hear what older lifestyle parents did with adult kids. we dont play in the house when the kids are even in the city much less the house. profiles are locked, devices are separate, never any mention of lifestyle stuff anywhere they could see. operationally we are clean. its the SOCIAL pattern thats the problem.