When will I be a BIG five?

So Joshua turned 5 on Monday the 5th - I know this because believe me when I say I was there! But on Tuesday he asked me... "When will I be a BIG 5?" I was very confused and thought he was talking about upper and lower case letters, so I told him numbers are all the same, no upper and lower case. To which he angrily replied, "NO! When will I be a BIG 5?" It took me a couple more minutes to realize and Joshua almost in tears to realize he was talking about his Birthday party, which wouldn't be until Saturday. As far as he was concerned he would not be a BIG 5 until his party. It didn't matter how much I tried to explain to him he was already 5, he would then say, "So I will be 6 on Saturday?" Finally, I conceded and said, "Yep, you're going to be a BIG 5 on Saturday". Sometimes you just can't win. So today, Sunday, Joshua is really and truly a BIG 5.