I have my periods after every 26 days. Which will be the best days of ovulation starting from the first day of periods?

The only possible way to know that is doing a daily Basal temp reading and charting it over the ENTIRE month...

Because women do NOT ovolate on a predictible schedule from woman to woman... you could theoretically put thirty women in a room, who have their periods in sync and they could all ovulate on different days. Reality is there would be more coomon days but not nearly enough one could claim it can be used to predict when anyone else would.