Learn Finnish: Onko sinulla kiire maaliskuussa tai huhtikuussa?

To say that something happens "in" a month, you add the ending -ssa (meaning "in" or "inside") directly to the name of the month. So maaliskuu (March) becomes maaliskuussa, and huhtikuu (April) becomes huhtikuussa.

Why does the Finnish sentence use sinulla (on you) to mean "are you"?

In Finnish, you don't say "I am busy". Instead, you express it as "I have a hurry". The word kiire means "hurry". To say someone "has" something, Finnish uses the "on" ending, -lla. So onko sinulla kiire literally translates to "is on you a hurry?", which is the standard way to ask "are you busy?".

How does Onko create a question here?

The word on means "is". When you attach the question particle -ko to the end of it, it turns a regular statement into a yes/no question. Sinulla on kiire is a statement meaning "You are busy", while Onko sinulla kiire? flips it to ask "Are you busy?".

Both months end in -kuussa. Will this be the same for the rest of the year?

Yes! Every month in Finnish ends with the word kuu, which means "moon" or "month". Because the word kuu has the back vowel "u", the ending will always be -ssa (never -ssä) for all twelve months of the year.