Argthtjtr

It was last Sunday.

Should I say, "When was your birthday?" or "When is your birthday?" to make a question for this sentence?

We usually ask When is your birthday? because the answer is a "constant" (same as What is your name?, it doesn't change from year to year).

But if you know perfectly well that the person you're asking had a birthday recently, it would be far more natural to ask when that was.

If you know (or suspect) that the person's birthday is "close" to "today", but you don't actually know whether it's in the recent past or the near future, this creates a bit of a problem, since whichever tense you choose has a fairly high probability of being "inappropriate". Many people would simply cover all bases by asking...

When is - or was - your birthday?

Usually one asks

When is your birthday?

Since one might be thinking of getting a present for the next birthday.

When was your birthday?

is used if you already know you missed their birthday and are seeking clarification.

If you're asking about the day/month of someone's birthday, then that doesn't change, so it's present tense. But in the example you gave, you are talking about a particular day that was their birthday (day/month/year). "Last Sunday" refers to 10 Feb 2019. That is a particular day that occurred in the past, so it takes past tense. If someone says "My birthday is 10 Feb", what they generally mean is "The tenth of February of every year is my birthday", and so that takes the present tense. "Birthday" can refer to the generic "day each year that has the same day/month as the day I was born", and that takes the present. But it can also refer to a specific day in a specific year that had the same day/month, and that takes the past (if it's already happened).

If you say

When was your birthday?

Then you are implicitly asking

When was your last birthday?

If you say

When is your birthday?

You are implicitly asking

When is your next birthday?

So ask either question, as you prefer. The response might be different. E.g.

My last birthday was December 25th.

Or

My birthday is the same day as Christmas.

If you would prefer one type of response, then ask in the way that encourages that response. Otherwise, both are correct unless they conflict with something. For example, if someone says

I just had my birthday.

It would make more sense to ask when it was, because you're discussing the most recent one. But even then, you can make is work. E.g.

Oh, so I'm safe from buying presents for a year? When is your next birthday?