When is Zacian coming in a box?
We'll probably get it in May/June.
Since XY had tins for Xerneas/Yveltal EX in March, Kalos Starters in June, Kanto Starters in October
Groudon/Kyogre EX in March, Hoenn Starters in June, Latios/Rayquaza/Hoopa EX in October
Gyarados/Machamp/Mewtwo EX in March, Xerneas/Yveltal/Zygarde EX in May, Volcanion/Magearna/Pikachu EX in September
Solgaleo/Lunala GX in March, Tapu Koko/Bulu GX in May, Ho-oh/Marshadow/Necrozma GX in October
Necrozma GX tins in March, Sylveon, Glaceon Leafeon GX in September (Skipped mid year tins)
Tag Team Tins were released in March and September
So far the first years of XY and SM had the legendaries and their starters as tins.
We got our Galar starters, so I assume its mixing it up since most of the time, the legends are first then the starters.
So around May/June is the trend
EDIT: Well Black and White had the starter tins before the legendary tins so there is precedent that a base set year would follow the same trends