Under California Code of Civil Procedure section 473(b), a party may bring a motion for relief from a "judgment, dismissal, order, or other proceeding taken against him or her through his or her mistake, inadvertence, surprise, or excusable neglect." Mistake under section 473(b) is not easily defined. (Cf. Varner, supra, 55 Cal.App.4th at p. 142 "less clear is the meaning and the nature of the 'mistake' which permits the setting aside of a judgment" under Fam. Code, 2122.) But the have noted under other circumstances--where the six months have passed for setting aside a judgment under section 473(b) and a judgment can only be set aside for extrinsic fraud or mistake--that "extrinsic mistake involves the excusable neglect of a party. " (In re Marriage of Melton (1994) 28 Cal.App.4th 931, 937.) Similarly, section 473(b) relief based upon a mistake of law has a reasonableness component: "An examination of the cases applying section 473 of the Code of Civil Procedure discloses that not every mistake of law is excusable but that an honest mistake is excusable, the determining factor being the reasonableness of the misconception ." (Viles v. State of California (1967) 66 Cal.2d 24, 29; cf Zamora v. Clayborn Contracting Group, Inc. (2002) 28 Cal.4th 249 attorney for party's mistake or inadvertence excusable if same error might have been made by reasonably prudent person under the same or similar circumstances.) Similarly, a party's neglect is excusable only if a reasonably prudent person in similar circumstances might have made the same error. (Bettencourt v. Los Rios Community College Dist. (1986) 42 Cal.3d 270, 276.) Likewise, a conclusory statement of inadvertence will not suffice: "The inadvertence referred to in section 473 is not inadvertence in the abstract but it must be inadvertence which is satisfactory. " (Davis v. Kay (1973) 34 Cal.App.3d 680, 683-684.) In addition to establishing that the judgment or order was entered as a result of mistake, inadvertence, surprise, or excusable neglect, the party seeking relief under section 473(b) "must also be diligent" in seeking that relief. (Zamora, supra, 28 Cal.4th at p. 258.) Thus, "in order to qualify for relief under section 473, the moving party must act diligently in seeking relief and must submit affidavits or testimony demonstrating a reasonable cause for the default. " (Elston, supra, 38 Cal.3d at p. 234.) For example, a party's unexplained delay of over three months after receipt of a default justified the denial of a belated motion for relief brought under section 473(b). (Ludka v. Memory Magnetics International (1972) 25 Cal.App.3d 316, 321-322; see also Caldwell v. Methodist Hospital (1994) 24 Cal.App.4th 1521, 1524-1525 unexplained delay of approximately three months from dismissal to filing of section 473(b) motion.) Furthermore, "the longer the delay in bringing the motion, the more substantial the justification for the delay must be in order for relief to be appropriately granted." (Stafford v. Mach (1998) 64 Cal.App.4th 1174, 1185.)