Valentines-element mosters are only available for breeding or purchase during the Valentine's season or in occasional out-of-season events such as the Anniversary Month Celebration. The Valentine's season is normally celebrated within the month of February, however the times vary from year to year. Check out the Special Occasions page for the dates and times of the past Valentine's seasons.
Although the Valentine's element appears on News articles when the Schmoocle is available for breeding/purchase, it does not appear under the Info section of the monster's profile like other confirmed elements. Thus, it is unknown if Seasonal elements are canon within the game.