S.N.
I lost my father 7 years ago and yes, the first few Father's Days were hard - but you must move on.
Now, I have heard that once BOTH of your parents are gone it is VERY hard b/c there is apparently an 'orphan' feeling that one has to move past.
It does seem a bit over board though that he is sleeping with their ashes and your mom is depressed for so long before and after... it appears the meds they are on and the person each of them is working with re: therapy need to be changed.
RE: what to do... I think it's too close to Mother's Day to do anything this year but you can definitely do something for Father's Day (and then do the same for MD next year w/ your mom). Not sure if the written word or spoken is better w/ your parents - do whichever you think would be best received.
Say/ write about what they mean to you but that those positive feelings/experiences are being diminished b/c of their not being able to move past this. Tell them that your wish is for YOUR children to have such beautiful memories of their grandparents - but due to what is happening - you don't think that is going to be the case...(only you can really word this for your family) then offer a solution ..... like the other poster mentioned. Create a NEW / POSITIVE ritual to do with your whole family on these days... something that you know they like to do.... and ask them directly - will you do this with us?
If they don't agree to it, then for your sake and for your children's sake (as this is a horrible example to be witnessing re: dealing with loss/grief) tell them that you will not be able to join them on this day (or the pre & post month's of depression). That you must provide your children with the tools to handle things like this (as death it is an inevitable occurrence) and that if they can't help you with that then you must do what you need to do for your family.
RE: your husband not honoring Mother's day - I'm sorry. The day is about honoring a mother's role in the family.... and that should cross all cultural boundaries. Again, I would ask or write a note to him and let him know that the day is important to you and that it's important to you that you raise children who understand and respect the role of a mother. If he refuses to do anything - well then YOU make it a special day for you and your children and when they are old enough - let them know that YOU did it b/c you want THEM to honor the women they marry/ know that they should be honored if you have daughters (no in the hallmark way of course... but in the 'simple and from the heart way).
You don't mention father's Day re: your Hubby... set the example there too. Do it for him so that your children learn to honor both parents/spouses. Best of luck to you.