How are the yield for Fullerton SGD Cash Fund calculated?

What you were looking at was the NAV Change %, which is not the annualized yield. Since they only provide NAV on working days, naturally NAV Change % is only published on working days. See the daily fund prices section here:

https://www.fullertonfund.com/fund/fullerton-sg...

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To calculate the actual per annum yield, you would need to calculate the NAV Change % over the last one year. However, this is just a snapshot of the actual annual yield on a particular day, and its value will change from day to day, albeit very small change for a cash fund like this.

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If you want to know annualized yield for a longer period, you can refer to the fund performance in their factsheet:

https://www.fullertonfund.com/documents/factshe...SGD_Cash_Fund-Class_A(SGD).PDF