During special events, you will gain summon points from conquests. You will need 100 of these points to perform a special event summon. You can perform 10 of these summons at a time. However, it is important to keep in mind that these summon points have an expiration date, usually set a few hours after the end of the related event.
Special event summons can give you normal, u-normal, rare or s-rare units. Your total number of conquests for the event increases your chances of getting more powerful units, so it may be advised to wait near the end pf the event before spending your points.