Typically (but by no means always) a mommy is the primary caregiver or Number One in the eyes of a toddler. Some toddlers' affection for the mommy to do everything can intensify around these months. Most mothers waffle between loving the fact that they are unconditionally adored by their children and having feelings of resentment that they are seen as the only ones wanted to . . . bathe the children, read a book, put the toddlers down for nap, prepare their meals, etc. No one will ever argue that primary caregiving is not hard, physical work.
And this is not easy on dads either. While we know that families are now swaying from typical work-home patterns, traditionally, dads are away from home during the day and look forward to spending time in the evening with their families. A dad may feel frustrated from the toddler's constant preference for his mommy. Fortunately, with most kids, dads' efforts are recognized in child development and he is awarded with another normal phase later in the preschool years where he is singled out as the adored adult in the child's eye.
In the meantime, Mom and Dad can work together on helping the child accept both parents as caregivers. Part of the reason the child prefers his mommy is because she may be the one throughout the bulk of the day who provides important primary caregiving—changing diapers, helping the child go to sleep, holding the child if he has an ouchie, preparing meals and snacks, etc.
At nighttime, it is important for Dad to step into this role too. For situations where the child will not accept Dad at all, perhaps Mommy needs to step out for a nighttime walk or dinner with some friends to help the dad and toddler establish their own nighttime routine. Also, Mom needs to resist the temptation to micromanage Dad's caregiving skills. Let him development his own style of caregiving. Maybe he'll make some mistakes—but the toddler will surely let him know the preferred way. Finally, invest in some books about daddies. Mom can read these books during the day and talk about Daddy coming home in the evening to spend time with the toddler.