Why the hell is it outside?
It's normal for parish patronal feast when for the Liturgy comes not only bishop, a lot of clergy and parishioners, but also other guests.
During the foot pilgrimage from Warsaw to Grabarka we have also one Divine Liturgy in a forest (of course with alta, icons, candles and a normal tent
Must be a Polish thing lol. Never seen that in Norway, Sweden, USA, or Germany. lol
Because foot pilgrimage from Warsaw to Grabarka goes more than 100 km through the territories there is not any Orthodox church and it's important to partake Holy Communion, we have blessing of our Metropolitan to once during the pilgrimage, in a special place in forest we can celebrate Divine Liturgy. I'm attaching a picture from this Liturgy.
I know that also in Serbia and Romania for great celebrations with a lot of clergy and laymen there is organised a special altar (a tent or even a building as Michał posted) before the church.
Maybe in Poland it's less unusual because Roman Catholics quite often celebrate theirs Masses outside the church