In some jurisdictions, the DOS is the date one spouse tells another that he or she wants a divorce; in others, it’s the date a spouse departs. In some jurisdictions, it is possible to legally separate and continue to live together under one roof as house mates.
In general there are three types of separation: trial, permanent and legal, and each has a potential to affect the legal rights of the parties. More to the point, the date of each is very important.
Some couples agree to a trial separation and while living apart decide to separate permanently and later divorce or reconcile. In a trial separation one door may be opened, but none have been closed. Sometimes couples separate to "sort out their feelings," and these separations may happen without the d-word of divorce even being mentioned. In many cases, nothing is put on paper in any way memorializing the separation.
A permanent separation means the spouses are living apart with the intention of divorce. The date of a permanent separation generally draws a line in the sand relative to property, income and debts. In this case, physical separation must be joined with an independent intent by at least one of the parties to divorce and communicated to the other. A preponderance of evidence is satisfactory to prove this.
A legal separation, which is available in some jurisdictions, confers a different legal status on the spouses who are not free to marry again.
Different jurisdictions establish different criteria for a separation, but the date can make a big difference in the value of property distributed as part of the divorce settlement. The separation date can have an impact on the active and passive appreciation of assets subject to distribution.
In most jurisdictions, in order to obtain a no-fault divorce a couple must live "separate and apart" for a specified period. However, some jurisdiction have held that couples can separate "under the same roof"; that is, they have made the decision to end the marriage, gone separate ways but remained domiciled in the same house, usually for economic reasons.
Depending on the jurisdiction, a brief reconciliation or even a night together for "old times" may reset the date of separation.
Salient here, however, is the DOS, after which the assets and liabilities of the spouses -- what they own and what they owe -- are seen in a different light and subject to distribution if they decide to divorce. Many states treat the date of separation as the classification date, after which newly earned assets cease being marital or community property and become instead separate property.
Good legal advice is very helpful.