...If I were to give advice to the unfaithful spouse about milestones it would be to go over and above with celebrations, gifts, words, and distractions. Start and end the day with an apology and state the intent to do all you can to make up for the holidays, birthdays, and memories that were and will always stay stolen from the betrayed spouse...
This response would be beneficial & healing under 2 conditions: (1) the affair lasted at least a year and impacted all those special days and (2) the betrayed spouse is open to attempts to celebrate those days. Each couple's experience is a little different and there are some BS's who feel little affect of the affair on major events and others who want a year of "buffer" before any attempt to recognize them is made.
But your encouragement to unfaithful spouse's is probably ALWAYS a good intent. They should be willing to go to great lengths to reclaim these celebrations for their marriage. If they let their spouse know their intent and have the green light to do so, go for it!
This leads me to think about a caution I give to betrayed spouses who are in the discovery phase (figuring out the truth about the affair). There is often great curiosity surrounding the ways an affair may have intruded on things like Christmas, birthdays, etc. But I have witnessed the turmoil caused by the knowledge of these details long after the affair is over. IMO, it is much better, if possible, to avoid the kind of details that sink their claws into these kinds of reminders. Some things are known without asking or digging, but the rest of it usually best left out of your inquiry. As you move into healing & recovery, curiosity about these things will diminish much more quickly than the memories of details you find out.