The secret I'm thinking of was my older sister's plan to leave her job in a bank in Hanoi and start her own small café. She told me about it about a year ago, a few months before she actually did it.
She told me late one night when we were both visiting our parents. She explained that she'd been saving money secretly for two years and had already found a small place to rent near the lake. But she made me promise not to tell anyone, especially our parents, until she had everything ready.
She had good reasons for keeping it quiet. Our parents are quite traditional and very proud that she works at a bank, which they see as a stable, respectable job. She knew they'd worry and try to talk her out of it, and she wanted to show them a complete, realistic plan rather than just an idea. She also needed time to make sure she could really afford it.
Keeping the secret was harder than I expected. Every time my parents talked about her career at dinner, I felt guilty, and once my mother asked me directly if my sister seemed happy at work, and I had to change the subject very quickly. I felt a bit like a spy. On the other hand, I felt honoured that she trusted me so much, and I was excited for her. When she finally told our parents, she showed them a business plan and photos of the café, and although they were shocked, they agreed to support her. Afterwards, she gave me free coffee for life, which I think was a fair reward.