Can someone explain why we use the locative case here for February and how we use it? I don't think there is a lesson on locative posted. Thanks.
The preposition "w" requires locative. It's quite logical when you think about - it indicates the location (in).
This is often true. Note that sometimes it takes accusative instead (but not with months).
Try googling the endings you know and you'll notice one has way more results than the others. You can also check wikitionary: https://en.wiktionary.org/wiki/ksi%C4%85%C5%BCka#Polish