According to ThyroidUK;
In women who are menstruating, their body temperature varies with the cycle; creating errors which can be avoided if the basal temperature is taken on days 2, 3, 4 and 5 of the cycle. Men and non-menstruating women may take their temperature on any day.
According to Mary Shoman;
Record the readings for three to five consecutive days. Women who still have their menstrual period should not test on the first five days of their period but can begin on day 5. Men, and girls and women who are not menstruating can test any time of the month.
Which is right if you are menstruating?