This is the 35th most frequent Hebrew word.
יום
"Day."
The word 'יום' is used here to denote the current day, with the prefix 'ה' indicating 'the' day, meaning today.
היום אני הולך לפארק.
Today I am going to the park.
In this sentence, 'יום' is used in conjunction with a specific day (Sunday), to indicate a particular day in the week.
ביום ראשון יש לי פגישה חשובה.
On Sunday, I have an important meeting.
Here, 'יום' is used to describe a past occurrence of a day that was characterized as wonderful.