Return the first day of the week, month, quarter or year using DATETRUNC
DATETRUNC(date_part, date, [start_of_week])
The date_part can be:
[start_of_week] is an optional parameter which can be any day of the week (i.e "Monday", "Tuesday", etc)
Return the first day of this year
DATETRUNC('year', TODAY())
Return the first day of the quarter
DATETRUNC('quarter', TODAY())
Return the first day of the month one year ago
DATETRUNC('month', TODAY() -365)
Some tips:
These calculations come in handy when finding the number of days in the month, quarter or year dynamically.