Forecasting with moving seasonality

Dear all, 

I am facing this challenge and I'm not sure how to approach it. 

I have to forecast the nr of incoming cases and my seasonality is affected by some fixed periods (like Christmas) and some variable periods (like eastern).

Due to this, comparing using for instance SAMEPERIODLASTYEAR would work for the fixed periods but not for others like eastern that every year occurs on different dates. 

I have already created on my calendar a column defining these periods for past and future dates.

I'm probably not the first one that has faced a similar situation and I was wondering if somebody could give me a hint on how to tackle this. 

Thanks in advance