Easter working hours,confused me

"We only work a half day today right?" I asked my colleague Pam,  who was busy looking for something from the office cabinet.

Today is Easter Saturday, the only day our library is open during the whole Easter holiday,  and it's my turn to work this weekend.

I hated it!

But somehow, I've learnt that our library only opens  a half day today.

"Who said we are only working a half day today? It's Saturday schedule, we should open a whole day,  eight hours....."Pam pulled her head out of that deep cabinet, watched me, puzzled.

"Really?" I answered,  with disbelieve.

I didn't bring my lunch with me this morning,  because I've learnt from someone,  or somewhere that we only open half a day today, definitely!

I keep searching for the answer of "who?" where?" I got the idea from, but still couldn't figure out the answer.

"Right!" I said to myself, as I walked out of our office,  to collect the new books displayed on the shelf since yesterday.

As I walked past our counter ,  I saw a big sign hanging on the front of sliding door:

"Easter holiday library open hours:.........20th of April, Saturday from 10 am to 1 pm......"

"Pam,  come here please!" turned around, I start to call her.

"Read the public notice please!"

"What?" She read , surprised.Then went online to check our schedule from our library's webpage.

"Someone had made a mistake, strange, this notice has hung there for three days,  but nobody realized there is a mistake......."

She took the sign down,  and corrected the mistake very quickly.

"So,  we have to work untill this afternoon 5pm, right?" I asked her with disappointment.

"I am sorry ," Pam answered me."unfortunately,the answer is 'Yes'".