The sentence basically says "This is March's first week".
I think that's why Duo insists on "of"… so you're not confused into thinking the postposition "ka" means "in".
I think this because I just spent ages trying to get my head around how the sentence must be structured with it meaning "in". But as soon as I realised it's actually just the usual possessive "ka," it all suddenly made sense.
I'm glad they only allowed "of March". It helped me a lot.